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.

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.

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.

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.

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".

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.

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.

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.

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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.

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.