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.

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.

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.

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.

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.

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.

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.

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:

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.

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.