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.

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.

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:

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.

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:

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.

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.

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:

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.

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.