Leetcode on My AI Digest

LeetCode MEDIUM 1922 Count Good Numbers Summary

Fri, 05 Jun 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Count Good Numbers
Topics: Math, Recursion

Problem gist

The problem asks for the number of length-n digit strings that satisfy two position rules:

Digits at even indices can be one of 0, 2, 4, 6, or 8.
Digits at odd indices can be one of 2, 3, 5, or 7.

Indices are zero-based, so index 0 is even. The string can start with 0; this is a digit string, not a normal integer representation. Because the answer can be enormous, return it modulo 1_000_000_007.

LeetCode MEDIUM 1143 Longest Common Subsequence Summary

Wed, 03 Jun 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Longest Common Subsequence
Topics: String, Dynamic Programming

Problem gist

Given two strings, the task is to find the length of the longest sequence of characters that appears in both strings in the same relative order.

The characters do not need to be contiguous. For example, "ace" is a subsequence of "abcde" because a, c, and e appear in order, even though there are skipped characters between them.

LeetCode MEDIUM 1094 Car Pooling Summary

Mon, 01 Jun 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Car Pooling
Topics: Array, Sorting, Heap (Priority Queue), Simulation, Prefix Sum

What the problem is really asking

A car has a fixed number of seats and only travels east. Each trip is described as [passengers, pickup, dropoff]: the passengers enter at pickup and leave at dropoff.

LeetCode MEDIUM 1011 Capacity To Ship Packages Within D Days Summary

Sat, 30 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Capacity To Ship Packages Within D Days
Topics: Array, Binary Search

Problem gist

There is a line of packages, each with a weight. A ship moves packages in the given order, and each day it can carry packages only until the total loaded weight reaches its capacity. The task is to find the smallest ship capacity that can move every package within days days.

LeetCode MEDIUM 833 Find And Replace in String Summary

Thu, 28 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Find And Replace in String
Topics: Array, Hash Table, String, Sorting

Problem gist

The input is a base string s and three parallel arrays:

indices[i] says where a possible replacement starts.
sources[i] says what text must already appear there.
targets[i] says what text to write if the source matches.

Every replacement is checked against the original string, not against a partially edited string. That “simultaneous” detail is the core of the problem. If s = "abcd", indices = [0, 2], sources = ["a", "cd"], and targets = ["eee", "ffff"], both checks happen on "abcd", so the answer is "eeebffff".

LeetCode MEDIUM 787 Cheapest Flights Within K Stops Summary

Tue, 26 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Cheapest Flights Within K Stops
Topics: Dynamic Programming, Depth-First Search, Breadth-First Search, Graph Theory, Heap (Priority Queue), Shortest Path

Problem gist

There are n cities, and each directed flight has a departure city, an arrival city, and a price. Given src, dst, and k, return the cheapest price from src to dst using at most k stops. If no such route exists, return -1.

LeetCode MEDIUM 662 Maximum Width of Binary Tree Summary

Sun, 24 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Maximum Width of Binary Tree
Topics: Tree, Depth-First Search, Breadth-First Search, Binary Tree

Problem gist

The input is the root of a binary tree. The task is to find the largest width across all levels.

The subtle part is how width is defined. It is not just the number of real nodes on a level. The width runs from the leftmost real node to the rightmost real node on that level, counting the missing null positions between them as if the tree were laid out like a complete binary tree.

LeetCode MEDIUM 658 Find K Closest Elements Summary

Fri, 22 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Find K Closest Elements
Topics: Array, Two Pointers, Binary Search, Sliding Window, Sorting, Heap (Priority Queue)

What the problem is asking

The input is already sorted. Given a target value x, the task is to return the k values that are closest to it, still in sorted order.

LeetCode MEDIUM 444 Sequence Reconstruction Summary

Wed, 20 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Sequence Reconstruction
Topics: Array, Graph Theory, Topological Sort

What the problem is asking

The input gives a target sequence nums and a list of shorter sequences. Each shorter sequence says, “these numbers must appear in this relative order.”

The task is not just to ask whether nums can be built from those constraints. It asks whether nums is the only shortest sequence that satisfies every constraint. If another sequence of the same shortest length can also satisfy the constraints, the answer is False.

LeetCode MEDIUM 442 Find All Duplicates in an Array Summary

Mon, 18 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Find All Duplicates in an Array
Topics: Array, Hash Table, Sorting

Problem gist

The input is an array nums of length n. Every value is in the range 1 through n, and each value appears either once or twice. The task is to return every value that appears twice.

LeetCode MEDIUM 416 Partition Equal Subset Sum Summary

Fri, 15 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Partition Equal Subset Sum
Topics: Array, Dynamic Programming

What the problem is really asking

The input is an array of positive integers. The task is to decide whether the array can be split into two subsets whose sums are exactly equal.

LeetCode HARD 3721 Longest Balanced Subarray II Summary

Thu, 14 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: HARD
Problem: Longest Balanced Subarray II
Topics: Array, Hash Table, Divide and Conquer, Segment Tree, Prefix Sum

Problem gist

The input is an integer array. A subarray is balanced when the number of distinct even values inside it equals the number of distinct odd values inside it. Repeated copies of the same number only count once, so [3, 2, 2, 5, 4] has two distinct odd values, 3 and 5, and two distinct even values, 2 and 4.

LeetCode 703-variant Kth Largest with Dynamic K Summary

Wed, 13 May 2026 00:00:00 +0000

Generated by Claude with claude-opus-4-7

Quick facts

Problem: Kth Largest Element in a Stream
Topics: Design, Heap (Priority Queue), Data Stream

Problem gist

Design a collection that supports two operations in any interleaving:

insert(num) — add a number to the collection.
find_kth_largest(k) — return the k-th largest element currently in the collection.

The twist versus the original LeetCode 703 is that k is a parameter of every query and may change from call to call. The classic min-heap-of-size-k solution relied on k being fixed at construction so it could safely discard everything below the running k-th largest. Once k can grow on a later call, no element is safe to throw away, and the whole strategy has to change.

LeetCode HARD Encode Any Input to JSON String Summary

Wed, 13 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: HARD
Problem: Custom LeetCode-style prompt: Encode Any Input to JSON String
Topics: Serialization, Depth-First Search, Hash Table, Graph, Design

Problem gist

The task is to write an encoder that accepts an input value and returns the same kind of string a normal JSON serializer would return:

LeetCode MEDIUM 371 Sum of Two Integers Summary

Wed, 13 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Sum of Two Integers
Topics: Math, Bit Manipulation

Problem gist

The task is to return the sum of two signed integers without using the + or - operators. The important constraint is not about math knowledge; it is about understanding how binary addition works under the hood.

LeetCode MEDIUM 402 Remove K Digits Summary

Wed, 13 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Remove K Digits
Topics: String, Stack, Greedy, Monotonic Stack

Problem gist

The problem gives a non-negative integer as a string and asks for the smallest possible number after removing exactly k digits. The answer must not keep unnecessary leading zeroes, and if every digit is removed or the remaining digits are all zeroes, the answer is "0".

LeetCode MEDIUM Two-Color Necklace Splitting Summary

Wed, 13 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Custom LeetCode-style prompt: Two-Color Necklace Splitting
Topics: Array, Sliding Window, Prefix Sum, Greedy, Math

Problem gist

This is a special case of the necklace splitting problem: two people, two bead types, and therefore at most two cuts. The input is a necklace represented as an array containing only X and Y. Suppose the full array has:

LeetCode MEDIUM 240 Search a 2D Matrix II Summary

Tue, 12 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Search a 2D Matrix II
Topics: Array, Binary Search, Divide and Conquer, Matrix

Problem gist

The matrix is sorted in two directions at once. Every row is sorted from left to right, and every column is sorted from top to bottom. Given a target value, the task is to decide whether the target appears anywhere in the matrix.

LeetCode MEDIUM 279 Perfect Squares Summary

Tue, 12 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Perfect Squares
Topics: Math, Dynamic Programming, Breadth-First Search

Problem gist

Given a positive integer n, the task is to return the fewest perfect square numbers whose sum is exactly n. A perfect square is a number such as 1, 4, 9, 16, and so on.

LeetCode MEDIUM 304 Range Sum Query 2D - Immutable Summary

Tue, 12 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Range Sum Query 2D - Immutable
Topics: Array, Design, Matrix, Prefix Sum

Problem gist

The class receives a fixed 2D matrix and must answer many rectangle-sum queries. Each query gives the top-left cell (row1, col1) and the bottom-right cell (row2, col2), and asks for the sum of every matrix value inside that inclusive rectangle.

LeetCode MEDIUM 981 Time Based Key-Value Store Summary

Tue, 12 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Time Based Key-Value Store
Topics: Hash Table, String, Binary Search, Design

Problem gist

Design a key-value store that remembers history. A normal hash map answers “what is the current value for this key?” This problem asks “what was the value for this key at this timestamp?”

LeetCode MEDIUM 103 Binary Tree Zigzag Level Order Traversal Summary

Mon, 11 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Difficulty: MEDIUM
Problem: Binary Tree Zigzag Level Order Traversal

Problem gist

The input is the root of a binary tree. The task is to return the node values level by level, but the direction alternates:

The root level is read from left to right.
The next level is read from right to left.
The next level flips back to left to right, and so on.

For example, a tree with levels [3], [9, 20], and [15, 7] should return three rows: [3], then [20, 9], then [15, 7].

LeetCode MEDIUM 153 Find Minimum in Rotated Sorted Array Summary

Mon, 11 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Find Minimum in Rotated Sorted Array
Main tags: Array, Binary Search

What the problem is really asking

The input array was originally sorted in ascending order, then rotated. For example, [0, 1, 2, 4, 5, 6, 7] might become [4, 5, 6, 7, 0, 1, 2].

LeetCode MEDIUM 179 Largest Number Summary

Mon, 11 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Largest Number
Main tags: Array, String, Greedy, Sorting

What the problem is really asking

The input is a list of non-negative integers. The task is to reorder them so that their decimal representations form the largest possible concatenated number, then return that number as a string.

LeetCode MEDIUM 209 Minimum Size Subarray Sum Summary

Mon, 11 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Minimum Size Subarray Sum
Topics: Array, Binary Search, Sliding Window, Prefix Sum

Problem gist

Given an array of positive integers nums and a positive integer target, find the shortest contiguous subarray whose sum is at least target. If no such subarray exists, return 0.

LeetCode MEDIUM 99 Recover Binary Search Tree Summary

Mon, 11 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Recover Binary Search Tree
Main tags: Tree, Depth-First Search, Binary Search Tree, Binary Tree

Problem gist

A binary search tree should produce values in sorted order when walked with inorder traversal: left subtree, current node, then right subtree. In this problem, exactly two node values were swapped by mistake. The tree shape is still the same, but the values break the BST ordering rule.

LeetCode HARD 778 Swim in Rising Water Summary

Sun, 10 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: HARD
Problem: Swim in Rising Water
Main tags: Array, Binary Search, Depth-First Search, Breadth-First Search, Union-Find, Heap (Priority Queue), Matrix

What the problem is really asking

The input is an n x n grid of elevations. At time t, the water level is also t, and a cell is swimmable only if its elevation is at most t.

LeetCode MEDIUM 1015 Smallest Integer Divisible by K Summary

Sun, 10 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Smallest Integer Divisible by K
Main tags: Hash Table, Math

What the problem is really asking

The input is a positive integer k. The task is to find the length of the smallest positive integer made only of the digit 1 that is divisible by k.

LeetCode MEDIUM 1838 Frequency of the Most Frequent Element Summary

Sun, 10 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Frequency of the Most Frequent Element
Main tags: Array, Binary Search, Greedy, Sliding Window, Sorting, Prefix Sum

What the problem is really asking

Given an array of positive integers and a budget k, one operation lets us increase one element by 1. The goal is to make some value appear as many times as possible after using at most k total increments.

LeetCode MEDIUM 767 Reorganize String Summary

Sun, 10 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Reorganize String
Main tags: Hash Table, String, Greedy, Heap, Counting

What the problem is really asking

The input is a string s. The task is to rearrange its characters so that no two adjacent characters are equal. If no such rearrangement exists, return the empty string. Any valid rearrangement is accepted.

LeetCode HARD 135 Candy Summary

Sat, 09 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: HARD
Problem: Candy
Main tags: Array, Greedy

What the problem is really asking

There are children standing in a line. Each child has a rating, and the goal is to give out the fewest total candies while obeying two rules:

every child gets at least one candy
any child with a higher rating than an adjacent child must get more candies than that neighbor

The important detail is that the comparison is only between neighbors. A child does not need more candy than every lower-rated child, only more than the children immediately to the left and right when their rating is higher.

LeetCode HARD 329 Longest Increasing Path in a Matrix Summary

Sat, 09 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: HARD
Problem: Longest Increasing Path in a Matrix
Main tags: Array, Dynamic Programming, Depth-First Search, Graph Theory, Topological Sort, Memoization, Matrix

What the problem is really asking

The input is a grid of numbers. From any cell, a path may move only up, down, left, or right, and each move must go to a strictly larger value. The task is to return the maximum number of cells that can appear in any such path.

LeetCode MEDIUM 131 Palindrome Partitioning Summary

Sat, 09 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Difficulty: MEDIUM
Problem: Palindrome Partitioning

Problem gist

The input is a string s. The task is to return every possible way to split s into non-empty substrings such that every chosen substring is a palindrome.

For example, "aab" can be split as ["a", "a", "b"] or ["aa", "b"]. Both are valid because every piece reads the same forward and backward. A split like ["a", "ab"] is invalid because "ab" is not a palindrome.

LeetCode MEDIUM 636 Exclusive Time of Functions Summary

Sat, 09 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Exclusive Time of Functions
Main tags: Array, Stack

What the problem is really asking

The input describes calls on a single-threaded CPU. Each log entry has a function id, an event type, and a timestamp:

function_id:start_or_end:timestamp

A function can call another function before it finishes. While the child function runs, the parent is paused. The goal is to return, for every function id, how much time that function spent actually executing, excluding time spent inside child calls.

LeetCode HARD 843 Guess the Word Summary

Fri, 08 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Difficulty: HARD

Problem: Guess the Word

Problem gist

This is an interactive guessing problem. There is a hidden six-letter word inside the given list, and the only way to learn about it is to call master.guess(word). The API returns how many positions match the secret exactly. For example, if the guess and secret have the same letter in positions 0, 2, and 5, the response is 3.

LeetCode MEDIUM 79 Word Search Summary

Fri, 08 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Difficulty: MEDIUM

Problem: Word Search

Problem gist

The problem gives a 2D grid of characters and a target word. The task is to decide whether the word can be formed by walking through adjacent grid cells. Each step may move up, down, left, or right, and the same cell cannot be used twice in the same word path.

LeetCode MEDIUM 80 Remove Duplicates from Sorted Array II Summary

Fri, 08 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Difficulty: MEDIUM

Problem: Remove Duplicates from Sorted Array II

Problem gist

The input is a sorted array nums. The task is to modify it in place so every distinct value appears at most two times, while keeping the original relative order of the remaining values. The function returns k, the length of the valid prefix. Anything after index k - 1 does not matter.

LeetCode MEDIUM 974 Subarray Sums Divisible by K Summary

Fri, 08 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Subarray Sums Divisible by K
Main tags: Array, Hash Table, Prefix Sum

What the problem is asking

Given an integer array nums and an integer k, count how many contiguous subarrays have a sum divisible by k.

The word “contiguous” is the key constraint. A valid answer must count ranges like nums[left:right + 1], not arbitrary subsets. The same element can participate in many different subarrays, and overlapping ranges are counted separately.

LeetCode MEDIUM 1101 The Earliest Moment When Everyone Become Friends Summary

Thu, 07 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: The Earliest Moment When Everyone Become Friends
Main tags: Array, Union-Find, Sorting

What the problem is really asking

There are n people labeled from 0 to n - 1. The input is a list of friendship events. Each event has a timestamp and two people, meaning those two people become direct friends at that moment.

LeetCode MEDIUM 210 Course Schedule II Summary

Thu, 07 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Course Schedule II
Main tags: Depth-First Search, Breadth-First Search, Graph Theory, Topological Sort

What the problem is really asking

There are numCourses courses labeled from 0 to numCourses - 1.

Each prerequisite pair [course, prerequisite] means the prerequisite must be taken before the course. The task is to return any valid order that takes every course, or return an empty list when no such order exists.

LeetCode MEDIUM 503 Next Greater Element II Summary

Thu, 07 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Next Greater Element II
Main tags: Array, Stack, Monotonic Stack

What the problem is really asking

The input is an integer array nums, but the array is circular. After the last index, the scan wraps back to index 0.

For each index i, the task is to find the first value that is strictly greater than nums[i] when walking forward through this circular array. If no greater value appears before returning to i, the answer for that index is -1.

LeetCode MEDIUM 777 Swap Adjacent in LR String Summary

Thu, 07 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Difficulty: MEDIUM
Problem: Swap Adjacent in LR String

Problem gist

The string contains three characters: L, R, and X. Think of X as an empty space. The only allowed moves are:

XL -> LX, which moves an L one position to the left.
RX -> XR, which moves an R one position to the right.

Given start and end, the task is to decide whether start can become end after any number of those moves.

LeetCode MEDIUM 122 Best Time to Buy and Sell Stock II Summary

Wed, 06 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Difficulty: MEDIUM
Problem: Best Time to Buy and Sell Stock II

Problem gist

The input is a list of stock prices where prices[i] is the price on day i. The goal is to make as much profit as possible by buying and selling any number of times.

LeetCode MEDIUM 139 Word Break Summary

Wed, 06 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Word Break
Main tags: String, Hash Table, Dynamic Programming, Trie

What the problem is really asking

The input is a string s and a dictionary of valid words. The task is to decide whether s can be split into one or more dictionary words. Words may be reused, so the question is not about consuming the dictionary; it is about whether every character in the string can be covered by valid dictionary chunks.

LeetCode MEDIUM 799 Champagne Tower Summary

Wed, 06 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Difficulty: MEDIUM
Problem: Champagne Tower

Problem gist

A tower of glasses is stacked in rows. Row 0 has one glass, row 1 has two glasses, and so on. Each glass can hold exactly one cup. If a glass receives more than one cup, the extra amount spills equally into the two glasses directly below it.

LeetCode MEDIUM 739 Daily Temperatures Summary

Tue, 05 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Difficulty: MEDIUM

Problem: Daily Temperatures

Problem gist

Given a list of daily temperatures, return a new list where each position says how many days must pass before a strictly warmer temperature appears. If no warmer future day exists, that position should be 0.

For example, if the temperatures are:

LeetCode MEDIUM 743 Network Delay Time Summary

Tue, 05 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Difficulty: MEDIUM
Problem: Network Delay Time

Problem gist

The input describes a directed, weighted graph. Each edge u -> v has a travel time w, and a signal starts at node k. The task is to return how long it takes until every node receives the signal. If some node can never receive it, return -1.

LeetCode MEDIUM 249 Group Shifted Strings Summary

Mon, 04 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Difficulty: MEDIUM

Problem: Group Shifted Strings

Problem gist

Each string can be shifted forward through the alphabet, wrapping from z to a. For example, shifting "abc" once gives "bcd", and shifting "az" once gives "ba". The task is to group strings that can become each other by applying the same shift amount to every character.

LeetCode MEDIUM 528 Random Pick with Weight Summary

Mon, 04 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Random Pick with Weight
Main tags: Array, Prefix Sum, Binary Search, Randomized

What the problem is really asking

The class receives an array of positive weights, where w[i] describes how likely index i should be picked. Calling pickIndex() should return one index at random, but not uniformly. Index i should be returned with probability:

LeetCode MEDIUM 151 Reverse Words in a String Summary

Sun, 03 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Difficulty: MEDIUM
Problem: Reverse Words in a String

Problem gist

The input is a string that contains words separated by spaces. The output should contain the same words in reverse order, with exactly one space between adjacent words and no leading or trailing spaces.

The important detail is that the function is not reversing letters inside each word. It is reversing the sequence of words. For example, the words ["the", "sky", "is", "blue"] become ["blue", "is", "sky", "the"].

LeetCode MEDIUM 236 Lowest Common Ancestor of a Binary Tree Summary

Sun, 03 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Difficulty: MEDIUM
Problem: Lowest Common Ancestor of a Binary Tree

Problem gist

Given the root of a binary tree and two existing nodes p and q, return their lowest common ancestor.

An ancestor can be the node itself. That detail matters: if p is above q, then p is the answer. “Lowest” means deepest in the tree, not smallest by value. This is a normal binary tree, so there is no useful ordering rule like there would be in a binary search tree.

LeetCode HARD 127 Word Ladder Summary

Sat, 02 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: HARD
Problem: Word Ladder
Main tags: Breadth-First Search, Hash Table, String, Graph

What the problem is really asking

The input gives a beginWord, an endWord, and a dictionary called wordList.

Each move can change exactly one letter, and every intermediate word must appear in wordList. The task is to return the number of words in the shortest valid transformation sequence from beginWord to endWord. If no such sequence exists, return 0.

LeetCode MEDIUM 380 Insert Delete GetRandom O(1) Summary

Fri, 01 May 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Insert Delete GetRandom O(1)
Main tags: Array, Hash Table, Math, Design, Randomized

What the problem is really asking

The task is to design a set-like data structure with three operations:

insert(val) adds a value only if it is not already present.
remove(val) deletes a value only if it is present.
getRandom() returns one currently stored value, with every stored value equally likely.

The important constraint is that all three operations should run in average O(1) time.

LeetCode HARD 224 Basic Calculator Summary

Thu, 30 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: HARD
Problem: Basic Calculator
Main tags: Math, String, Stack, Recursion

What the problem is really asking

The input is a string expression that contains non-negative integers, +, -, parentheses, and spaces. The task is to return the final numeric value without using Python’s built-in expression evaluator.

LeetCode MEDIUM 155 Min Stack Summary

Wed, 29 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Min Stack
Main tags: Stack, Design

What the problem is really asking

The API looks simple: push, pop, top, and getMin.

The real requirement is hidden in getMin: it must return the current minimum in constant time, not by scanning the whole stack.

LeetCode MEDIUM 102 Binary Tree Level Order Traversal Summary

Tue, 28 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Binary Tree Level Order Traversal
Main tags: Tree, Breadth-First Search, Binary Tree

What the problem is really asking

The task is to read a binary tree one layer at a time.

Instead of visiting nodes in a depth-first order such as root, left subtree, right subtree, the problem wants the tree grouped by depth:

LeetCode MEDIUM 3453 Separate Squares I Summary

Mon, 27 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Separate Squares I
Main tags: Array, Binary Search

What the problem is really asking

This problem looks geometric, but the core idea is much simpler than it first appears.

The task is to place a horizontal line y = cut so that:

LeetCode MEDIUM 518 Coin Change II Summary

Sun, 26 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Coin Change II
Main tags: Array, Dynamic Programming

What the problem is really asking

The input gives a target amount and a list of coin denominations. Each denomination can be used as many times as needed.

The task is to count how many distinct combinations can make the amount exactly. It is not asking for the minimum number of coins, and it is not asking whether the amount is possible at all.

LeetCode MEDIUM 322 Coin Change Summary

Sat, 25 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Coin Change
Main tags: Array, Dynamic Programming, Breadth-First Search

What the problem is really asking

The input gives a set of coin denominations and a target amount. Each coin can be used as many times as needed.

The goal is not to count how many different ways the amount can be formed. The goal is to find the fewest coins needed to reach the exact amount. If the amount cannot be formed exactly, the answer is -1.

LeetCode HARD 239 Sliding Window Maximum Summary

Fri, 24 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: HARD
Problem: Sliding Window Maximum
Main tags: Array, Queue, Sliding Window, Heap (Priority Queue), Monotonic Queue

What the problem is really asking

The input gives an array and a window size k. Imagine placing a length-k window over the first k elements, recording the maximum value inside that window, then moving the window one step to the right and repeating until the end.

LeetCode MEDIUM 1004 Max Consecutive Ones III Summary

Fri, 24 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Max Consecutive Ones III
Main tags: Array, Binary Search, Sliding Window, Prefix Sum

What the problem is really asking

This problem sounds like a bit-flipping question, but the useful reframe is simpler:

find the longest contiguous subarray that contains at most k zeroes.

LeetCode MEDIUM 152 Maximum Product Subarray Summary

Fri, 24 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Maximum Product Subarray
Main tags: Array, Dynamic Programming

What the problem is really asking

This problem asks for the contiguous subarray whose product is as large as possible.

That sounds close to Maximum Subarray, but products behave very differently from sums. With sums, a very negative running total is usually bad news. With products, a very negative running product can suddenly become the best thing to keep if the next number is also negative.

LeetCode MEDIUM 221 Maximal Square Summary

Fri, 24 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Maximal Square
Main tags: Array, Dynamic Programming, Matrix

What the problem is really asking

The input is a grid of "0" and "1" values. The job is to find the area of the largest square made entirely of 1s.

The part that usually traps people is the word “square.” A brute-force approach tends to ask:

LeetCode MEDIUM 994 Rotting Oranges Summary

Fri, 24 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Rotting Oranges
Main tags: Array, Breadth-First Search, Matrix

Problem gist

The grid has three kinds of cells: empty (0), fresh oranges (1), and rotten oranges (2). Every minute, any fresh orange touching a rotten orange in the four main directions also becomes rotten.

LeetCode HARD 410 Split Array Largest Sum Summary

Thu, 23 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: HARD
Problem: Split Array Largest Sum
Main tags: Array, Binary Search, Dynamic Programming, Greedy, Prefix Sum

What the problem is really asking

The input array must be cut into exactly k non-empty, contiguous pieces. Each cut creates one subarray, and the score of a split is the largest subarray sum that appears anywhere in that split.

LeetCode MEDIUM 540 Single Element in a Sorted Array Summary

Thu, 23 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Single Element in a Sorted Array
Main tags: Array, Binary Search

What the problem is really asking

The array is sorted, and every value appears exactly twice except for one value that appears once.

If the array were not sorted, the quickest idea would be XOR: identical values cancel out, so the final XOR is the answer. That works in O(n) time and O(1) space.

LeetCode MEDIUM 912 Sort an Array Summary

Thu, 23 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5.4

Difficulty: MEDIUM
Problem: Sort an Array

Problem gist

The task is deliberately simple to state: given an integer array, return the same values in ascending order. The interview signal is not whether someone knows Python’s built-in sort, but whether they can implement a sorting strategy with clear time and space guarantees.

LeetCode HARD 85 Maximal Rectangle Summary

Wed, 22 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: HARD
Problem: Maximal Rectangle
Main tags: Array, Dynamic Programming, Stack, Matrix, Monotonic Stack

What the problem is really asking

The matrix contains only "0" and "1", and the goal is to find the area of the largest axis-aligned rectangle made entirely of "1" cells.

LeetCode MEDIUM 189 Rotate Array Summary

Wed, 22 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5.4

Quick facts

Difficulty: MEDIUM
Problem: Rotate Array
Main tags: Array, Math, Two Pointers

What the problem is really asking

The problem gives an array nums and an integer k. The task is to rotate the array to the right by k positions, modifying the original list in place.

LeetCode MEDIUM 207 Course Schedule Summary

Wed, 22 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Course Schedule
Main tags: Depth-First Search, Breadth-First Search, Graph Theory, Topological Sort

What the problem is really asking

There are numCourses courses labeled from 0 to numCourses - 1.

Each prerequisite pair [a, b] means course b must be finished before course a.

LeetCode MEDIUM 78 Subsets Summary

Wed, 22 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Subsets
Main tags: Array, Backtracking, Bit Manipulation

What the problem is really asking

Given an array nums of distinct integers, return every possible subset of those numbers.

A subset can contain none of the numbers, some of the numbers, or all of the numbers. The empty subset [] is valid, and the full array is valid too. Because the input numbers are distinct, there is no duplicate-handling problem in the base version.

LeetCode HARD 84 Largest Rectangle in Histogram Summary

Tue, 21 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: HARD
Problem: Largest Rectangle in Histogram
Main tags: Array, Stack, Monotonic Stack

What the problem is really asking

Each bar in the histogram has width 1, and the goal is to find the largest possible rectangle that can be formed by taking one or more adjacent bars.

LeetCode MEDIUM 146 LRU Cache Summary

Tue, 21 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: LRU Cache
Main tags: Hash Table, Linked List, Design, Doubly-Linked List

What the problem is really asking

This problem is not mainly about storing key-value pairs. It is about building a data structure that can answer two different questions at the same time:

LeetCode MEDIUM 162 Find Peak Element Summary

Tue, 21 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Find Peak Element
Main tags: Array, Binary Search

What the problem is really asking

This problem does not ask for the maximum value in the whole array.

It only asks for any index whose value is greater than its neighbors. That is a much weaker requirement, and that weaker requirement is exactly what makes the binary-search solution possible.

LeetCode MEDIUM 238 Product of Array Except Self Summary

Tue, 21 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Product of Array Except Self
Main tags: Array, Prefix Sum

What the problem is really asking

For each index, the answer should be the product of every number except the one currently sitting at that index.

If the input is [1, 2, 3, 4], then:

LeetCode MEDIUM 424 Longest Repeating Character Replacement Summary

Tue, 21 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Longest Repeating Character Replacement
Main tags: Hash Table, String, Sliding Window

What the problem is really asking

The input is a string and a budget k. One operation lets us replace any single character with any other character. The goal is to find the longest substring that can be turned into a string of all one repeated character using at most k replacements.

LeetCode HARD 1944 Number of Visible People in a Queue Summary

Mon, 20 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: HARD
Problem: Number of Visible People in a Queue
Main tags: Array, Stack, Monotonic Stack

What the problem is really asking

Each person is looking to the right. For every position, the task is to count how many people are visible before the view is blocked.

LeetCode HARD 354 Russian Doll Envelopes Summary

Mon, 20 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: HARD
Problem: Russian Doll Envelopes
Main tags: Array, Binary Search, Dynamic Programming, Sorting

What the problem is really asking

Each envelope has a width and a height. One envelope can go inside another only if both dimensions are strictly smaller.

So the real question is:

LeetCode MEDIUM 167 Two Sum II - Input Array Is Sorted Summary

Mon, 20 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Two Sum II - Input Array Is Sorted
Main tags: Array, Two Pointers, Binary Search

What the problem is really asking

The input array is already sorted in non-decreasing order, and the goal is to find the two numbers whose sum equals the target.

LeetCode MEDIUM 200 Number of Islands Summary

Mon, 20 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Number of Islands
Main tags: Array, Depth-First Search, Breadth-First Search, Union-Find, Matrix

What the problem is really asking

The grid is a map of water and land:

"1" means land
"0" means water

An island is one connected piece of land using only the four cardinal directions: up, down, left, and right.

LeetCode MEDIUM 215 Kth Largest Element in an Array Summary

Mon, 20 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Kth Largest Element in an Array
Main tags: Array, Divide and Conquer, Sorting, Heap (Priority Queue), Quickselect

What the problem is really asking

This problem asks for the value that would appear in position k if the array were sorted in descending order.

LeetCode MEDIUM 253 Meeting Rooms II Summary

Mon, 20 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Meeting Rooms II
Main tags: Array, Two Pointers, Greedy, Sorting, Heap (Priority Queue)

What the problem is really asking

Each interval represents one meeting with a start time and an end time.

The question is not really about building a calendar or labeling rooms. It is asking:

LeetCode MEDIUM 347 Top K Frequent Elements Summary

Mon, 20 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Top K Frequent Elements
Main tags: Array, Hash Table, Bucket Sort, Heap

What the problem is really asking

The input is an array of integers and a number k.

The task is not to sort the array itself. It is to:

LeetCode MEDIUM 875 Koko Eating Bananas Summary

Mon, 20 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Koko Eating Bananas
Main tags: Array, Binary Search

What the problem is really asking

Koko has several piles of bananas and exactly h hours before the guards return. In one hour, she chooses one pile and eats up to k bananas from that pile. If the pile has fewer than k, she finishes it and the rest of that hour is wasted.

LeetCode HARD 212 Word Search II Summary

Sun, 19 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: HARD
Problem: Word Search II
Main tags: Array, String, Backtracking, Trie, Matrix

What the problem is really asking

The input gives a letter grid and a list of candidate words. A word is valid if it can be traced through horizontally or vertically adjacent cells without reusing the same cell inside that one word.

LeetCode HARD 2402 Meeting Rooms III Summary

Sun, 19 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: HARD
Problem: Meeting Rooms III
Main tags: Array, Sorting, Heap (Priority Queue), Simulation
Frequency in source list: 17.6

What the problem is really asking

This is not the classic “how many meeting rooms are needed?” question.

The number of rooms is fixed up front, and every meeting has to be assigned under three rules:

LeetCode HARD 312 Burst Balloons Summary

Sun, 19 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: HARD
Problem: Burst Balloons
Main tags: Array, Dynamic Programming

What the problem is really asking

This problem looks like a simulation question, but the hard part is not the simulation itself. The hard part is that every burst changes the future.

When balloon i is burst, the coins earned are:

LeetCode HARD 407 Trapping Rain Water II Summary

Sun, 19 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: HARD
Problem: Trapping Rain Water II
Main tags: Array, Breadth-First Search, Heap (Priority Queue), Matrix
Frequency in source list: 17.6

What the problem is really asking

This is the 2D version of Trapping Rain Water.

Instead of one row of bars, there is a full grid of elevations. After rain falls, water can sit on low cells, but only if every path from that cell to the outer border is blocked by walls that are high enough.

LeetCode MEDIUM 138 Copy List with Random Pointer Summary

Sun, 19 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Copy List with Random Pointer
Main tags: Hash Table, Linked List

What the problem is really asking

Each node in the list has two outgoing pointers:

next, which gives the usual linked-list order
random, which can point to any node in the list or to None

The task is to build a deep copy, not just another pointer to the same nodes.

LeetCode MEDIUM 29 Divide Two Integers Summary

Sun, 19 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Divide Two Integers
Main tags: Math, Bit Manipulation

What the problem is really asking

Two integers are given: dividend and divisor.

The task is to return the integer quotient of dividend / divisor, but with three constraints that make the problem interesting:

LeetCode MEDIUM 36 Valid Sudoku Summary

Sun, 19 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Valid Sudoku
Main tags: Array, Hash Table, Matrix

What the problem is really asking

This problem is not asking to solve the Sudoku board.

It is only asking whether the current partially filled board is still legal. That means every filled digit must satisfy three rules at the same time:

LeetCode HARD 76 Minimum Window Substring Summary

Sat, 18 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: HARD
Problem: Minimum Window Substring
Main tags: Hash Table, String, Sliding Window

What the problem is really asking

There are two strings:

s, the big string where the answer must come from
t, the set of required characters the answer must contain

The task is to return the shortest substring of s that contains every character from t, including duplicates.

LeetCode MEDIUM 237 Delete Node in a Linked List Summary

Sat, 18 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Delete Node in a Linked List
Main tags: Linked List

What the problem is really asking

This problem sounds impossible the first time it is heard.

In a normal singly linked list, deleting a node means changing the previous node’s next pointer so it skips the node being removed. But this problem does not give the head of the list or the previous node. It gives only the node that should disappear, and it guarantees that this node is not the tail.

LeetCode MEDIUM 417 Pacific Atlantic Water Flow Summary

Sat, 18 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Pacific Atlantic Water Flow
Main tags: Array, Depth-First Search, Breadth-First Search, Matrix

What the problem is really asking

Each cell in the grid is a height. Water can flow from a cell to one of its four neighbors if the neighbor is at the same height or lower.

LeetCode MEDIUM 494 Target Sum Summary

Sat, 18 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Target Sum
Main tags: Array, Dynamic Programming, Backtracking
Frequency in source list: 22.1

What the problem is really asking

Each number in nums must receive either a plus sign or a minus sign.

After choosing a sign for every element, the final expression must evaluate to target.

LeetCode MEDIUM 61 Rotate List Summary

Sat, 18 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Rotate List
Main tags: Linked List, Two Pointers
Frequency in source list: 25.6

What the problem is really asking

The input is a singly linked list and a non-negative integer k.

Rotating the list to the right by k means taking the last k nodes, preserving their order, and moving them to the front.

LeetCode MEDIUM 8 String to Integer (atoi) Summary

Sat, 18 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: String to Integer (atoi)
Main tags: String

What the problem is really asking

The input is a string, and the task is to extract the integer prefix that follows a very specific parsing rule:

ignore leading spaces
read at most one optional + or -
read as many consecutive digits as possible
stop as soon as the next character is not a digit
clamp the result into the 32-bit signed integer range

So this is not a math problem. It is a parsing problem with a few edge cases that must be handled in the correct order.

LeetCode MEDIUM 90 Subsets II Summary

Sat, 18 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Subsets II
Main tags: Array, Backtracking, Bit Manipulation

What the problem is really asking

This is the power set problem with one extra constraint that changes everything: the input can contain duplicate values, but the output cannot contain duplicate subsets.

If the array were all distinct, the job would be straightforward. At each position, either include the number or skip it, and every path through that decision tree would produce a unique subset.

LeetCode MEDIUM 907 Sum of Subarray Minimums Summary

Sat, 18 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Sum of Subarray Minimums
Main tags: Array, Dynamic Programming, Stack, Monotonic Stack
Frequency in source list: 22.1

What the problem is really asking

For every contiguous subarray, take its minimum value. Then add all of those minimums together.

That sounds simple until the number of subarrays is considered. An array of length n has about n^2 / 2 subarrays, so listing each one and recomputing its minimum quickly becomes too slow.

LeetCode HARD 124 Binary Tree Maximum Path Sum Summary

Fri, 17 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: HARD
Problem: Binary Tree Maximum Path Sum
Main tags: Dynamic Programming, Tree, Depth-First Search, Binary Tree

What the problem is really asking

This problem is easy to misread at first.

The path does not have to start at the root, and it does not have to end at a leaf. It can start anywhere and end anywhere, as long as it follows parent-child edges and does not reuse a node.

LeetCode HARD 44 Wildcard Matching Summary

Fri, 17 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: HARD
Problem: Wildcard Matching
Main tags: String, Dynamic Programming, Greedy

What the problem is really asking

The pattern contains two special characters:

? matches exactly one character
* matches any sequence of characters, including the empty sequence

The match must cover the entire string. That is the part that makes the problem interesting.

LeetCode HARD 60 Permutation Sequence Summary

Fri, 17 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: HARD
Problem: Permutation Sequence
Main tags: Math, Recursion

What the problem is really asking

The numbers 1 through n can form n! different permutations in lexicographic order.

The problem asks for the kth permutation in that ordering without generating all the earlier ones first.

LeetCode HARD 68 Text Justification Summary

Fri, 17 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: HARD
Problem: Text Justification
Main tags: Array, String, Simulation

What the problem is really asking

The input is a list of words and a fixed line width. The task is to break the words into lines and return the exact text layout that a formatter would produce.

LeetCode MEDIUM 24 Swap Nodes in Pairs Summary

Fri, 17 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Swap Nodes in Pairs
Main tags: Linked List, Recursion, Iteration, Pointer Manipulation

What the problem is really asking

The input is the head of a singly linked list. Every adjacent pair of nodes must be swapped:

1 -> 2 -> 3 -> 4 becomes 2 -> 1 -> 4 -> 3
1 -> 2 -> 3 becomes 2 -> 1 -> 3

The important constraint is that node values cannot be copied around. The solution has to rewire the list itself.

LeetCode MEDIUM 287 Find the Duplicate Number Summary

Fri, 17 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Find the Duplicate Number
Main tags: Array, Two Pointers, Binary Search

What the problem is really asking

The input has length n + 1, but every value is guaranteed to be in the range 1 through n.

That guarantee means one value must appear more than once. So the duplicate is not the hard part. The hard part is satisfying the interview constraints at the same time:

LeetCode MEDIUM 399 Evaluate Division Summary

Fri, 17 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Evaluate Division
Main tags: Graph, DFS, BFS, Union-Find

What the problem is really asking

Each equation like a / b = 2.0 gives a multiplicative relationship between two variables.

The query x / y asks:

can the known equations connect x to y, and if they can, what ratio do those equations imply?

LeetCode MEDIUM 50 Pow(x, n) Summary

Fri, 17 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Pow(x, n)
Main tags: Math, Recursion

What the problem is really asking

The task is to compute x^n, but the real constraint is not the arithmetic itself. It is that n can be very large, including negative values, so a straightforward loop that multiplies x by itself |n| times is too slow.

LeetCode HARD 25 Reverse Nodes in k-Group Summary

Thu, 16 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: HARD
Problem: Reverse Nodes in k-Group
Main tags: Linked List, Recursion

What the problem is really asking

The list must be processed in consecutive chunks of size k.

If a full chunk exists, those k nodes should be reversed in place. If fewer than k nodes remain at the end, that tail must stay exactly as it is.

LeetCode MEDIUM 137 Single Number II Summary

Thu, 16 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Single Number II
Main tags: Array, Bit Manipulation
Core requirement: linear time and constant extra space

What the problem is really asking

The array contains one value that appears exactly once, while every other value appears exactly three times. The obvious Counter or hash-map solution works, but it uses extra memory, so it does not satisfy the strongest version of the problem.

LeetCode MEDIUM 45 Jump Game II Summary

Thu, 16 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Jump Game II
Main tags: Array, Dynamic Programming, Greedy

What the problem is really asking

Each position tells how far forward it can jump. The goal is not to maximize distance on each move. The goal is to reach the last index using as few jumps as possible.

LeetCode MEDIUM 57 Insert Interval Summary

Thu, 16 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Insert Interval
Main tag: Array

What the problem is really asking

The input intervals are already sorted by start time, and they do not overlap with each other.

That matters because the problem is not asking to rebuild the whole interval set from scratch. It is asking where one new interval fits into an already clean timeline.

LeetCode MEDIUM 73 Set Matrix Zeroes Summary

Thu, 16 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Set Matrix Zeroes
Main tags: Array, Hash Table, Matrix

What the problem is really asking

The rule sounds simple:

if any cell contains 0, then every cell in that cell’s row and every cell in that cell’s column must become 0.

LeetCode HARD 32 Longest Valid Parentheses Summary

Wed, 15 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: HARD
Problem: Longest Valid Parentheses
Main tags: String, Dynamic Programming, Stack

What the problem is really asking

The input is a string made only of ( and ).

The task is not to check whether the whole string is valid. The task is to find the length of the longest contiguous substring that forms a valid parentheses expression.

LeetCode HARD 41 First Missing Positive Summary

Wed, 15 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: HARD
Problem: First Missing Positive
Main tags: Array, Hash Table

What the problem is really asking

Given an unsorted integer array, return the smallest positive integer that does not appear in the array.

The tricky part is not the definition of the answer. The tricky part is the constraint: the best solution must run in O(n) time and use only O(1) extra space.

LeetCode MEDIUM 12 Integer to Roman Summary

Wed, 15 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Integer to Roman
Main tags: Hash Table, Math, String

What the problem is really asking

The input is an integer between 1 and 3999, and the output must be its Roman numeral form.

The main challenge is not basic lookup. The hard part is handling the subtractive cases correctly:

LeetCode MEDIUM 130 Surrounded Regions Summary

Wed, 15 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Surrounded Regions
Main tags: Array, Depth-First Search, Breadth-First Search, Union-Find, Matrix

What the problem is really asking

The board contains X and O. The task is to flip only the O cells that belong to regions fully enclosed by X.

The important phrase is “surrounded region.” A region is safe if it can reach the border through adjacent O cells. A region is captured only if there is no such path.

LeetCode MEDIUM 16 3Sum Closest Summary

Wed, 15 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: 3Sum Closest
Main tags: Array, Two Pointers, Sorting

What the problem is really asking

The input is an integer array and a target value. The job is to pick exactly three numbers whose sum is as close to the target as possible.

LeetCode MEDIUM 176 Second Highest Salary Summary

Wed, 15 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Second Highest Salary
Main tags: Database, SQL

What the problem is really asking

There is an Employee table with a salary column. The task is to return the second-highest distinct salary value.

That word “distinct” is the whole trick.

If the largest salary appears many times, it still counts as only one rank. So this is not:

LeetCode MEDIUM 19 Remove Nth Node From End of List Summary

Wed, 15 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Remove Nth Node From End of List
Main tags: Linked List, Two Pointers

What the problem is really asking

The input is a singly linked list and a positive integer n.

The task is to delete the node that is n positions from the end and return the new head.

LeetCode HARD 10 Regular Expression Matching Summary

Tue, 14 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: HARD
Problem: Regular Expression Matching
Main tags: String, Dynamic Programming, Recursion

What the problem is really asking

This problem is not asking for a full regex engine.

It asks for one very specific kind of full-string match:

. matches exactly one arbitrary character
* means “repeat the immediately previous token zero or more times”
the whole string must match the whole pattern

That last rule is the detail that makes many first attempts fail. A match is not enough if there are leftover characters in either the text or the pattern.

LeetCode HARD 37 Sudoku Solver Summary

Tue, 14 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: HARD
Problem: Sudoku Solver
Main tags: Array, Hash Table, Backtracking, Matrix

What the problem is really asking

This problem is a constraint satisfaction problem disguised as a puzzle.

The board is almost complete already. The task is to fill each . with a digit so that three rules are always true:

LeetCode MEDIUM 300 Longest Increasing Subsequence Summary

Tue, 14 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Longest Increasing Subsequence
Main tags: Array, Binary Search, Dynamic Programming

What the problem is really asking

An array of integers is given. The task is to find the length of the longest subsequence whose values are strictly increasing.

The key word is subsequence, not subarray.

LeetCode MEDIUM 39 Combination Sum Summary

Tue, 14 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Combination Sum
Main tags: Array, Backtracking

What the problem is really asking

The input gives a set of distinct candidate numbers and a target. The task is to return every unique combination of candidates whose sum is exactly the target.

Two details define the real problem:

LeetCode MEDIUM 6 Zigzag Conversion Summary

Tue, 14 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Zigzag Conversion
Main tags: String

What the problem is really asking

The problem gives a string and a row count, then asks for the string that would appear if the characters were written in a zigzag pattern and read row by row.

LeetCode MEDIUM 62 Unique Paths Summary

Tue, 14 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Unique Paths
Main tags: Math, Dynamic Programming, Combinatorics

What the problem is really asking

There is an m x n grid. A robot starts in the top-left corner and wants to reach the bottom-right corner. The robot can only move right or down.

LeetCode MEDIUM 74 Search a 2D Matrix Summary

Tue, 14 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Search a 2D Matrix
Main tags: Array, Binary Search, Matrix

What the problem is really asking

The matrix looks two-dimensional, but the key rule makes it behave like one long sorted array:

each row is sorted from left to right
the first value in a row is greater than the last value in the previous row

So the task is not really “search a grid.” It is “search a sorted sequence” while remembering that the sequence happens to be stored in matrix form.

LeetCode MEDIUM 904 Fruit Into Baskets Summary

Tue, 14 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Fruit Into Baskets
Main tags: Array, Hash Table, Sliding Window

What the problem is really asking

The story sounds specific, but the core problem is simple:

find the length of the longest contiguous subarray that contains at most two distinct values.

LeetCode HARD 23 Merge k Sorted Lists Summary

Mon, 13 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: HARD
Problem: Merge k Sorted Lists
Main tags: Linked List, Divide and Conquer, Heap (Priority Queue), Merge Sort

What the problem is really asking

Each input list is already sorted. The task is to combine all of them into one final sorted linked list without losing that order.

LeetCode MEDIUM 17 Letter Combinations of a Phone Number Summary

Mon, 13 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Letter Combinations of a Phone Number
Main tags: Hash Table, String, Backtracking

What the problem is really asking

Each digit from 2 to 9 maps to a small set of letters, just like an old phone keypad. Given a string of digits, the task is to return every possible letter string that could be formed by choosing one mapped letter for each digit.

LeetCode MEDIUM 18 4Sum Summary

Mon, 13 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: 4Sum
Main tags: Array, Two Pointers, Sorting

What the problem is really asking

The surface task is to return every unique quadruplet whose sum equals target.

The harder part is not the arithmetic. It is avoiding duplicate answers while still searching efficiently.

LeetCode MEDIUM 46 Permutations Summary

Mon, 13 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Permutations
Main tags: Array, Backtracking

What the problem is really asking

The input contains distinct integers, and the task is to return every possible ordering of those integers.

That means the algorithm is not trying to find one best answer. It is trying to systematically explore a decision tree where:

LeetCode MEDIUM 48 Rotate Image Summary

Mon, 13 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Rotate Image
Main tags: Array, Math, Matrix

What the problem is really asking

The input is an n x n matrix, and the task is to rotate it 90 degrees clockwise.

The important constraint is that the rotation must happen in place. That means the matrix itself must be rearranged without allocating another full n x n matrix for the final answer.

LeetCode MEDIUM 72 Edit Distance Summary

Mon, 13 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Edit Distance
Main tags: String, Dynamic Programming

What the problem is really asking

Two strings are given, word1 and word2.

The goal is to transform word1 into word2 using as few operations as possible. The allowed operations are:

insert one character
delete one character
replace one character

This is not a greedy string-matching problem. A locally good edit can make the rest of the string worse, so the real challenge is deciding which prefixes of the two words should be matched together.

LeetCode MEDIUM 75 Sort Colors Summary

Mon, 13 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Sort Colors
Main tags: Array, Two Pointers, Sorting

What the problem is really asking

The input array contains only 0, 1, and 2. Those values represent three categories, and the goal is to reorder the array in place so all 0s come first, then all 1s, then all 2s.

LeetCode HARD 51 N-Queens Summary

Sun, 12 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: HARD
Problem: N-Queens
Main tags: Array, Backtracking

What the problem is really asking

The board details can make this problem look more complicated than it is.

At a higher level, the task is:

place exactly one queen in each row
never reuse a column
never place two queens on the same diagonal
return every full placement that satisfies those rules

That “one queen per row” reframing is the key simplification.

LeetCode MEDIUM 198 House Robber Summary

Sun, 12 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: House Robber
Main tags: Array, Dynamic Programming

What the problem is really asking

Each house has some amount of money. The thief wants the largest total possible, but robbing two neighboring houses triggers the alarm.

So the real question is:

At every house, is it better to:

LeetCode MEDIUM 22 Generate Parentheses Summary

Sun, 12 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Generate Parentheses
Main tags: String, Dynamic Programming, Backtracking

What the problem is really asking

Given n pairs of parentheses, generate every string that is:

exactly 2n characters long
made only of ( and )
balanced at every point

The last condition is the important one.

LeetCode MEDIUM 33 Search in Rotated Sorted Array Summary

Sun, 12 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Search in Rotated Sorted Array
Main tags: Array, Binary Search

What the problem is really asking

The array started out sorted in ascending order, then got rotated at some pivot.

That means a sequence like [0, 1, 2, 4, 5, 6, 7] might become [4, 5, 6, 7, 0, 1, 2].

LeetCode MEDIUM 34 Find First and Last Position of Element in Sorted Array Summary

Sun, 12 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Find First and Last Position of Element in Sorted Array
Main tags: Array, Binary Search

What the problem is really asking

The input array is sorted, and the target value may appear many times in one contiguous block.

The task is not just to decide whether the target exists. It is to return the exact starting and ending indices of that block.

LeetCode MEDIUM 49 Group Anagrams Summary

Sun, 12 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Group Anagrams
Main tags: Array, Hash Table, String, Sorting

What the problem is really asking

Two strings belong in the same group if they use the exact same letters with the exact same counts, just in a different order.

So the problem is not really about comparing every string against every other string. It is about finding a stable “signature” for each word, so all words with the same signature land in the same bucket.

LeetCode MEDIUM 54 Spiral Matrix Summary

Sun, 12 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Spiral Matrix
Main tags: Array, Matrix, Simulation

What the problem is really asking

The input is a 2D matrix. The task is to return every value exactly once in spiral order:

go left to right across the top
then top to bottom down the right side
then right to left across the bottom
then bottom to top up the left side
keep repeating until there is nothing left

This is a traversal problem. Nothing in the matrix needs to be modified. The only challenge is knowing when to turn and when a side has already been fully consumed.

LeetCode MEDIUM 55 Jump Game Summary

Sun, 12 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Jump Game
Main tags: Array, Dynamic Programming, Greedy

What the problem is really asking

Each position tells how far the player is allowed to jump from that spot.

The goal is not to find the minimum number of jumps. It is only to decide whether the last index is reachable at all.

LeetCode MEDIUM 128 Longest Consecutive Sequence Summary

Sat, 11 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Longest Consecutive Sequence
Main tags: Array, Hash Table, Union-Find

What the problem is really asking

The input is an unsorted integer array. The task is to find the length of the longest run of values that can be arranged as x, x + 1, x + 2, ....

LeetCode MEDIUM 15 3Sum Summary

Sat, 11 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: 3Sum
Main tags: Array, Two Pointers, Sorting

What the problem is really asking

The input is an integer array, and the goal is to return every unique triplet whose values add up to 0.

Two details make the problem harder than it looks:

LeetCode MEDIUM 31 Next Permutation Summary

Sat, 11 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Next Permutation
Main tags: Array, Two Pointers

What the problem is really asking

The array represents one permutation of the numbers.

The task is to mutate it into the very next permutation in lexicographic order, which means:

make the number just a little larger
keep that increase as small as possible
do everything in place

If the current permutation is already the largest possible one, such as [3, 2, 1], then there is no larger answer. In that case, the correct result is the smallest permutation, which is ascending order.

LeetCode MEDIUM 394 Decode String Summary

Sat, 11 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Decode String
Main tags: String, Stack, Recursion

What the problem is really asking

The input is an encoded string where patterns like 3[ab] mean “repeat ab three times.” The tricky part is that the repeated part can itself contain more encoded pieces, such as 3[a2[c]].

LeetCode MEDIUM 53 Maximum Subarray Summary

Sat, 11 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Maximum Subarray
Main tags: Array, Divide and Conquer, Dynamic Programming

What the problem is really asking

The input is an integer array, and the task is to choose one contiguous subarray whose sum is as large as possible.

The important constraint is contiguity. This is not “pick the best numbers anywhere in the array.” Once a starting point is chosen, the subarray must be one uninterrupted block.

LeetCode MEDIUM 56 Merge Intervals Summary

Sat, 11 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Merge Intervals
Main tags: Array, Sorting

What the problem is really asking

The input is a list of inclusive intervals like [start, end]. The goal is to combine every set of overlapping intervals and return the smallest list of non-overlapping intervals that covers the same ranges.

LeetCode MEDIUM 7 Reverse Integer Summary

Sat, 11 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Reverse Integer
Main tags: Math

What the problem is really asking

Given a signed 32-bit integer, reverse its decimal digits.

Examples:

123 becomes 321
-123 becomes -321
120 becomes 21

The catch is that the answer must still fit inside the signed 32-bit range:

LeetCode HARD 4 Median of Two Sorted Arrays Summary

Fri, 10 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: HARD
Problem: Median of Two Sorted Arrays
Main tags: Array, Binary Search, Divide and Conquer

What the problem is really asking

Two arrays are already sorted. If they were merged into one larger sorted array, the task would be to return the middle value.

LeetCode HARD 42 Trapping Rain Water Summary

Fri, 10 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: HARD
Problem: Trapping Rain Water
Main tags: Array, Two Pointers, Dynamic Programming, Stack

What the problem is really asking

The input is an elevation map where each number is the height of a vertical bar with width 1.

After rain falls, water can sit in the valleys between taller bars. The task is to compute the total amount of water that stays trapped.

LeetCode MEDIUM 11 Container With Most Water Summary

Fri, 10 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Container With Most Water
Main tags: Array, Two Pointers, Greedy

What the problem is really asking

The input is an array of heights. Each height represents a vertical line drawn at that index.

The task is to pick two lines that, together with the x-axis, form a container that holds the most water.

LeetCode MEDIUM 2 Add Two Numbers Summary

Fri, 10 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Add Two Numbers
Main tags: Linked List, Math, Recursion

What the problem is really asking

Two linked lists represent two non-negative integers. Each node stores one digit, and the digits are already in reverse order, so the ones place comes first.

LeetCode MEDIUM 3 Longest Substring Without Repeating Characters Summary

Fri, 10 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Longest Substring Without Repeating Characters
Main tags: Hash Table, String, Sliding Window

What the problem is really asking

Given a string, the goal is to find the length of the longest contiguous block of characters with no repeats.

The important word is contiguous. This is not asking for a subsequence that can skip characters. It is asking for one unbroken window inside the string.

LeetCode MEDIUM 5 Longest Palindromic Substring Summary

Fri, 10 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Longest Palindromic Substring
Frequency: 64.1
Main tags: Two Pointers, String, Dynamic Programming

What the problem is really asking

Given a string, return the longest contiguous substring that reads the same forward and backward.

The word “substring” is the important constraint. This is not about picking scattered characters. The answer has to be one continuous block inside the original string.

LeetCode MEDIUM 560 Subarray Sum Equals K Summary

Fri, 10 Apr 2026 00:00:00 +0000

Generated by Codex with GPT-5

Quick facts

Difficulty: MEDIUM
Problem: Subarray Sum Equals K
Main tags: Array, Hash Table, Prefix Sum

What the problem is really asking

The input is an integer array and a target k.

The task is not to find the longest subarray or to return the subarray itself. It is simply to count how many contiguous subarrays add up to k.