LeetCode

This section collects Hot100 greedy tutorials, focusing on which local state is enough, why it stays valid while scanning, and how that local choice proves the global answer. Recommended Reading Order LeetCode 121: Best Time to Buy and Sell Stock, derive one-transaction greedy from the historical minimum price LeetCode 55: Jump Game, use the farthest reachable range to decide reachability LeetCode 45: Jump Game II, upgrade reachability into minimum jump layers LeetCode 435: Non-overlapping Intervals, turn minimum removals into maximum kept intervals LeetCode 452: Minimum Number of Arrows, derive interval greedy from shared intersections

This section collects Hot100 Union-Find templates and problems, focusing on set representatives, path compression, merge conditions, connectivity checks, and connected component counting.

This section collects Hot100 Trie tutorials, focusing on node fields, child traversal, end markers, and loop invariants.

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. ...

Subtitle / Summary This is Hot100 article #1 for the series: Subarray Sum Equals K. We reduce the naive O(n^2) approach to O(n) with prefix sum plus a frequency hash map, then map the same pattern to real engineering scenarios. Reading time: 12-15 min Tags: Hot100, prefix sum, hash map SEO keywords: Subarray Sum Equals K, prefix sum, hash map, O(n), Hot100 Meta description: O(n) counting of subarrays with sum k using prefix sum + hash map, with complexity analysis and runnable multi-language code. Target Readers Hot100 learners who want stable reusable templates Intermediate engineers who want to transfer counting patterns to real data pipelines Interview prep readers who want to master prefix sum + hash map Background / Motivation “Count subarrays whose sum equals k” is one of the most classic counting problems. It appears in log analytics, risk threshold hits, and transaction sequence statistics. The two-loop brute force method is straightforward, but slows down quickly as input grows. So we need an O(n) method that scales. ...

Subtitle / Summary The key is not comparing values, but comparing node identity (same object / same address). This ACERS guide explains the naive hash approach, the length-alignment approach, and the most practical switch-head two-pointer template, with runnable multi-language implementations under the no-modification and no-cycle constraints. Reading time: 10-14 min Tags: Hot100, linked list, two pointers SEO keywords: Intersection of Two Linked Lists, switch heads, O(1) space, LeetCode 160 Meta description: Two pointers walk A then B and B then A, guaranteeing meeting at the intersection or both reaching null within m+n steps, with O(m+n) time and O(1) space. A - Algorithm (Problem and Algorithm) Problem Restatement Given heads headA and headB of two singly linked lists, return the node where they intersect. If they do not intersect, return null. ...

Subtitle / Summary The constraint “the path can start anywhere, but must go downward” makes root-to-leaf DP insufficient. This ACERS guide explains prefix sums on trees: convert any downward path into a difference of two prefix sums, maintain a frequency hash map during one DFS, and finish in O(n). Reading time: 12–15 min Tags: binary tree, prefix sum, DFS, hash map SEO keywords: Path Sum III, tree prefix sum, prefix-sum hash, LeetCode 437 Meta description: Count downward paths whose sum equals targetSum in O(n) via prefix sum + hash map, with derivation, tradeoffs, and multi-language implementations. A — Algorithm (Problem & Algorithm) Problem Restatement Given the root root of a binary tree and an integer targetSum, return the number of downward paths whose node values sum to targetSum. The path does not need to start at the root or end at a leaf, but it must go downward (parent → child). ...

Subtitle / Summary Spiral traversal looks like “just printing in a fancy order”, but the real difficulty is getting boundaries and invariants right. This ACERS guide gives a reusable boundary-shrinking template and runnable multi-language solutions. Reading time: 12–15 min Tags: Hot100, matrix, simulation, boundary shrinking SEO keywords: Hot100, Spiral Matrix, clockwise traversal, boundary shrinking, LeetCode 54 Meta description: O(mn) spiral order traversal using boundary shrinking, with pitfalls, engineering scenarios, and runnable code. A — Algorithm Problem Restatement Given an m × n matrix matrix, return all elements in clockwise spiral order. ...

Problem Requirement You are given n non-negative integers in height. Each integer is the height of a bar with width 1, and all bars are adjacent from left to right. After rain, taller bars on both sides may hold water above shorter bars. Return the total amount of water trapped by the entire elevation map. LeetCode expects this interface: class Solution: def trap(self, height: List[int]) -> int: ... Example 1 Input: height = [0,1,0,2,1,0,1,3,2,1,2,1] Output: 6 Example 2 Input: height = [4,2,0,3,2,5] Output: 9 Constraints n == len(height) 1 <= n <= 2 * 10^4 0 <= height[i] <= 10^5 Step 1: First Answer How Much Water One Position Holds Do not calculate the whole elevation map yet. Focus on one position: ...

Subtitle / Summary Maximum Subarray is the classic 1D DP / greedy template. This ACERS guide explains Kadane’s idea, engineering use cases, and runnable multi-language solutions. Reading time: 10–12 min Tags: Hot100, dynamic programming, greedy SEO keywords: Maximum Subarray, Kadane, dynamic programming, O(n), Hot100 Meta description: Kadane O(n) maximum subarray sum with engineering scenarios and multi-language code. A — Algorithm Problem Restatement Given an integer array nums, find the contiguous subarray with the largest sum (must contain at least one element) and return the sum. ...

Subtitle / Summary A classic event-spacing validation model. This ACERS guide explains the one-pass logic, engineering use cases, and runnable multi-language solutions. Reading time: 10–12 min Tags: array, two pointers, event spacing SEO keywords: LeetCode 1437, event spacing, O(n) Meta description: One-pass validation for minimum spacing between 1s, with engineering use cases and multi-language code. A — Algorithm Problem Restatement Given an integer array nums and integer k, return true if every pair of 1s is at least k apart; otherwise return false. ...

Subtitle / Summary A classic bit-manipulation template: determine if a number is a power of two in O(1). This ACERS guide covers the core insight, practical uses, and runnable multi-language implementations. Reading time: 8–12 min Tags: bit manipulation, binary, math SEO keywords: Power of Two, bit manipulation, binary, O(1), LeetCode 231 Meta description: O(1) power-of-two check using bit tricks, with engineering scenarios and multi-language code. A — Algorithm Problem Restatement Given an integer n, determine whether it is a power of two. Return true if it is; otherwise, return false. ...

Subtitle / Summary A standard fixed-window counting problem. This ACERS guide explains the sliding-window model, engineering use cases, and runnable multi-language solutions. Reading time: 10–12 min Tags: sliding window, string, fixed window SEO keywords: Maximum Number of Vowels, Sliding Window, Fixed Window Meta description: Fixed-window sliding count for maximum vowels with engineering applications. A — Algorithm Problem Restatement Given a string s and an integer k, return the maximum number of vowels in any substring of length k. ...