Leetcode

Subtitle / Summary LeetCode 23 is a k-way merge problem, not just repeating LeetCode 21 in a loop. This ACERS guide derives the optimal structure, explains tradeoffs between divide-and-conquer and min-heap, and provides runnable implementations in multiple languages. Reading time: 12-16 min Tags: Hot100, linked list, divide and conquer, merge SEO keywords: Merge K Sorted Lists, LeetCode 23, divide and conquer, O(N log k), Hot100 Meta description: A full ACERS explanation of Merge K Sorted Lists from naive ideas to O(N log k) divide-and-conquer, with engineering mapping and multi-language code. A - Algorithm (Problem and Algorithm) Problem Restatement Given an array lists of k sorted linked lists, merge them into one sorted linked list and return it. ...

Subtitle / Summary LeetCode 142 upgrades cycle detection into cycle entry localization. The robust template is Floyd: first detect a meeting inside the cycle, then reset one pointer to head and move both by one step; the next meeting node is the cycle entry. Reading time: 12-16 min Tags: Hot100, linked list, fast slow pointers, Floyd SEO keywords: Linked List Cycle II, cycle entry, Floyd, fast slow pointers, O(1) space, LeetCode 142, Hot100 Meta description: Floyd cycle detection + entry localization with proof intuition, engineering mapping, and runnable multi-language implementations in O(n) time and O(1) extra space. A - Algorithm (Problem and Algorithm) Problem Restatement Given head of a singly linked list, return the node where the cycle begins. If there is no cycle, return null. ...

Subtitle / Summary This problem is the linked-list version of merge-sort’s merge step. Use a sentinel node plus two pointers to splice nodes in ascending order in O(m+n), without rebuilding the list. Reading time: 10-12 min Tags: Hot100, linked list, merge, two pointers SEO keywords: Merge Two Sorted Lists, sentinel node, linked list merge, LeetCode 21, Hot100 Meta description: A complete ACERS guide for LeetCode 21 with derivation, correctness invariants, pitfalls, and runnable multi-language code. A - Algorithm (Problem and Algorithm) Problem Restatement Given heads list1 and list2 of two sorted linked lists, merge them into one sorted linked list and return its head. The merged list should be formed by splicing together nodes from the original lists. ...

Subtitle / Summary First Missing Positive is a classic in-place indexing problem. Place each valid value x into slot x-1, then scan for the first mismatch. This ACERS guide explains the derivation, invariant, pitfalls, and production-style transfer. Reading time: 12-15 min Tags: Hot100, array, in-place hashing SEO keywords: First Missing Positive, in-place hashing, index mapping, O(n), Hot100, LeetCode 41 Meta description: O(n)/O(1) solution for First Missing Positive using in-place index placement, with complexity analysis, engineering scenarios, and runnable multi-language code. Target Readers Hot100 learners building stable array templates Intermediate developers who want to master in-place indexing techniques Engineers who need linear-time, constant-space array normalization Background / Motivation “Find the smallest missing positive” is fundamentally a placement problem. ...

Subtitle / Summary LeetCode 25 is the upgrade path from 206 (full reversal) and 92 (interval reversal): split by groups, reverse inside each full group, reconnect safely, and keep the last incomplete group unchanged. Reading time: 14-18 min Tags: Hot100, linked list, group reversal, dummy node SEO keywords: Reverse Nodes in k-Group, group reversal, LeetCode 25, Hot100 Meta description: In-place k-group linked-list reversal with dummy-node anchoring and safe reconnection, including pitfalls, complexity, and runnable multi-language implementations. A - Algorithm (Problem and Algorithm) Problem Restatement Given the head of a linked list and an integer k, reverse nodes in groups of size k and return the modified head. If the number of remaining nodes is less than k, keep them in original order. You must modify pointers, not just node values. ...

Subtitle / Summary Reorder List is a classic pointer choreography problem: find middle, reverse second half, then merge alternately. This guide derives the in-place O(n)/O(1) method from naive ideas and turns it into a reusable Hot100 template. Reading time: 12-15 min Tags: Hot100, linked list, in-place SEO keywords: Reorder List, split reverse merge, LeetCode 143, O(1) space Meta description: A full ACERS explanation of Reorder List with correctness intuition, boundary handling, engineering mapping, and runnable code in Python/C/C++/Go/Rust/JS. A - Algorithm (Problem and Algorithm) Problem Restatement Given the head of a singly linked list head, reorder it to: ...

Subtitle / Summary Reverse Linked List II is not about full-list reversal; it is about reversing a strict middle interval while preserving both outer connections. This ACERS guide explains the dummy-node anchor, head-insertion loop, and boundary-safe implementation. Reading time: 12-15 min Tags: Hot100, linked list, sublist reversal, dummy node SEO keywords: Reverse Linked List II, sublist reversal, dummy node, head insertion, LeetCode 92, Hot100 Meta description: In-place sublist reversal with dummy node + head insertion in O(n)/O(1), with correctness intuition, pitfalls, and runnable multi-language code. A - Algorithm (Problem and Algorithm) Problem Restatement Given the head of a singly linked list and two integers left and right (1 <= left <= right <= n), reverse the nodes from position left to right, and return the new head. ...

Subtitle / Summary Detecting a cycle in a linked list is a pointer chasing problem, not a value comparison problem. This ACERS guide explains why Floyd’s fast/slow pointers must meet if a cycle exists, how to avoid null-pointer bugs, and how the same pattern maps to engineering checks. Reading time: 10-12 min Tags: Hot100, linked list, fast slow pointers, Floyd SEO keywords: Linked List Cycle, Floyd, fast slow pointers, LeetCode 141, Hot100 Meta description: O(n)/O(1) cycle detection in singly linked lists using Floyd fast/slow pointers, with alternatives, common mistakes, and runnable multi-language code. A - Algorithm (Problem and Algorithm) Problem Restatement Given head node head of a singly linked list, determine whether there is a cycle in the list. Return true if a cycle exists, else false. ...

Subtitle / Summary The core of palindrome validation is symmetric comparison, but a singly linked list cannot move backward. The most stable engineering template is: find middle -> reverse second half in-place -> compare -> reverse back to restore. Reading time: 10-14 min Tags: Hot100, linked list, fast slow pointers, in-place reverse SEO keywords: Palindrome Linked List, fast slow pointers, reverse second half, O(1) space, LeetCode 234 Meta description: O(n)/O(1) palindrome check for singly linked list with middle detection, second-half reversal, comparison, and full structure restoration. A - Algorithm (Problem and Algorithm) Problem Restatement Given the head of a singly linked list head, return true if it is a palindrome; otherwise return false. ...

Subtitle / Summary Reverse Linked List is the first serious pointer-rewiring exercise in Hot100. It looks simple, but most bugs come from broken links and wrong operation order. This ACERS guide explains the three-pointer iterative template thoroughly and compares it with recursion. Reading time: 10-12 min Tags: Hot100, linked list, pointer, iteration SEO keywords: Hot100, Reverse Linked List, three pointers, iterative, recursive, LeetCode 206 Meta description: Three-pointer iterative reversal in O(n)/O(1), with recursive contrast, common pitfalls, engineering mapping, and runnable multi-language implementations. A - Algorithm (Problem and Algorithm) Problem Restatement Given the head of a singly linked list, reverse the list and return the new head. ...