Generated by Codex with GPT 5.6 Sol XHigh

Quick facts

• Difficulty: MEDIUM
• Problem: Reorder List
• Topics: Linked List, Two Pointers, Stack, Recursion

Problem gist

The input is a singly linked list in its ordinary front-to-back order:

L0 → L1 → L2 → ... → Ln

It must be rearranged by alternating nodes from the front and back:

Generated by Codex with GPT 5.6 Sol XHigh

Quick facts

• Difficulty: MEDIUM
• Problem: Linked List Cycle II
• Topics: Hash Table, Linked List, Two Pointers

Problem gist

A singly linked list may eventually point back to an earlier node instead of ending at None. The task is to return the exact node where that cycle begins, or None if the list has no cycle. The list must not be modified.

Generated by Codex with GPT 5.6 Sol XHigh

Quick facts

• Difficulty: MEDIUM
• Problem: Populating Next Right Pointers in Each Node
• Topics: Linked List, Tree, Depth-First Search, Breadth-First Search, Binary Tree

Problem gist

Each node in a perfect binary tree has an extra next pointer. The task is to make that pointer lead to the node immediately to its right on the same level, or to None when the node is the level’s last one.

Generated by Codex with GPT 5.6 Sol High

Quick facts

• Difficulty: HARD
• Problem: Custom LeetCode-style prompt: Count Nodes in a Distributed Tree
• Topics: Distributed Systems, Tree, Asynchronous Programming, State Machine, Idempotency

Problem gist

Every tree node runs in a separate process or on a separate machine. A node knows only its parent and its direct children, and it can communicate with them only by sending asynchronous messages.

Generated by Codex with GPT 5.6 Sol XHigh

Quick facts

• Difficulty: MEDIUM
• Problem: Construct Binary Tree from Preorder and Inorder Traversal
• Topics: Array, Hash Table, Divide and Conquer, Tree, Binary Tree

Problem gist

Two arrays describe the same binary tree:

• Preorder visits each node before its children: root, left subtree, right subtree.
• Inorder visits each node between its children: left subtree, root, right subtree.

The task is to rebuild and return the tree. Every value is unique, and the two arrays are guaranteed to describe the same tree.

Generated by Codex with GPT 5.6 Sol XHigh

Quick facts

• Difficulty: MEDIUM
• Problem: Longest Non-Decreasing Subarray After Replacing at Most One Element
• Topics: Array, Dynamic Programming

Problem gist

Given an integer array, one element may be replaced with any integer. The goal is to maximize the length of a contiguous subarray whose values never decrease from left to right.

Generated by Codex with GPT 5.6 Sol XHigh

Quick facts

• Difficulty: HARD
• Problem: Minimum Time to Finish the Race
• Topics: Array, Dynamic Programming

Problem gist

Each tire type is described by [f, r]. A fresh tire completes its first lap in f seconds, but keeping it for consecutive laps makes it geometrically slower: its next lap times are f, f * r, f * r^2, and so on. There is an unlimited supply of every tire type. Between laps, the driver may spend changeTime seconds to put on any fresh tire.

Generated by Codex with GPT 5.6 Sol XHigh

Quick facts

• Difficulty: HARD
• Problem: Partition Array Into Two Arrays to Minimize Sum Difference
• Topics: Array, Two Pointers, Binary Search, Dynamic Programming, Bit Manipulation, Sorting, Ordered Set, Bitmask

Problem gist

The input contains 2 * n integers. Every number must go into one of two arrays, and each array must contain exactly n numbers. The goal is to make the absolute difference between the two array sums as small as possible.

Generated by Codex with GPT 5.6 Sol XHigh

Quick facts

• Difficulty: MEDIUM
• Problem: Minimize Maximum Pair Sum in Array
• Topics: Array, Two Pointers, Greedy, Sorting

Problem gist

An even-length array must be divided into pairs, with every element used exactly once. Each pair has a sum, and the score of a complete pairing is its largest pair sum. The goal is to make that worst pair as small as possible.

Generated by Codex with GPT 5.6 Sol XHigh

Quick facts

• Difficulty: MEDIUM
• Problem: The k-th Lexicographical String of All Happy Strings of Length n
• Topics: String, Backtracking

Problem gist

A happy string uses only a, b, and c, and no two neighboring characters may be equal. The task is to consider every happy string of length n in lexicographic order and return the k-th one. If fewer than k such strings exist, the answer is the empty string.