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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].

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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].

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.

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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.

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.

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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.

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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.

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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.

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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.

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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.