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
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 131. Palindrome Partitioning is not hard because of recursion alone. The real challenge is separating two concerns clearly: where the next cut starts, and whether the current substring is a valid palindrome. Reading time: 15-18 min Tags: Hot100, backtracking, string, palindrome, DP SEO keywords: Palindrome Partitioning, backtracking, palindrome DP, string partition, LeetCode 131 Meta description: Learn LeetCode 131 by building the solution from scratch, using suffix-based DFS plus a precomputed palindrome table. A — Algorithm Problem Restatement Given a string s, partition it so that every substring in the partition is a palindrome. Return all possible palindrome partitionings of s. ...
Subtitle / Summary 216. Combination Sum III is where backtracking adds one more important constraint: not only must the sum match, but the combination length must be exactly k. That turns the problem into a clean fixed-length combination search over the bounded range 1..9. Reading time: 12-15 min Tags: backtracking, fixed length, pruning, combination search SEO keywords: Combination Sum III, fixed-length backtracking, k numbers, pruning, LeetCode 216 Meta description: Build the stable solution for LeetCode 216 from scratch by understanding exact-length backtracking, sorted pruning, and the bounded candidate set 1..9. A — Algorithm Problem Restatement Find all valid combinations of k numbers that add up to n, subject to these rules: ...
Subtitle / Summary If 39. Combination Sum teaches “reuse is allowed”, then 40. Combination Sum II teaches the next real upgrade: duplicate values exist, each number may be used at most once, and de-duplication must happen at the correct tree level. Reading time: 14-16 min Tags: backtracking, dedup, pruning, combination search SEO keywords: Combination Sum II, backtracking, same-layer dedup, pruning, LeetCode 40 Meta description: Build the stable solution for LeetCode 40 from scratch by understanding sorted pruning, use-once recursion, and the same-layer duplicate skip rule. A — Algorithm Problem Restatement Given a collection of candidate numbers candidates and a target integer target, return all unique combinations where the chosen numbers sum to target. ...
Subtitle / Summary Combination Sum is the first Hot100 backtracking problem that really mixes three ideas at once: combination-style search, a running target, and safe pruning after sorting. The point is not to jump straight to the template, but to build it step by step from the problem itself. Reading time: 14-16 min Tags: Hot100, backtracking, combination sum, pruning SEO keywords: Combination Sum, backtracking, pruning, remain, DFS Meta description: Learn LeetCode 39 by building the solution from scratch, using path, remain, repeated candidate reuse, and sorted pruning. A — Algorithm Problem Restatement Given an array of distinct integers candidates and a target integer target, return a list of all unique combinations of candidates where the chosen numbers sum to target. ...
Subtitle / Summary If Subsets teaches the skeleton of combination-style backtracking, Permutations teaches the core of state-based backtracking: at each position, choose one unused element, continue until the path length reaches n, and only then collect the answer. Reading time: 10-12 min Tags: Hot100, backtracking, permutations, DFS SEO keywords: Permutations, backtracking, used array, DFS, LeetCode 46 Meta description: Learn the stable permutation backtracking template for LeetCode 46, with state recovery, engineering analogies, and runnable multi-language solutions. A — Algorithm Problem Restatement Given an array nums of distinct integers, return all possible permutations. The answer may be returned in any order. ...
Subtitle / Summary Clone Graph is not a traversal-only problem. The real challenge is preserving graph structure while avoiding duplicate copies in the presence of cycles. The stable solution is a traversal plus a hash map from original nodes to cloned nodes. Reading time: 12-15 min Tags: graph, dfs, bfs, hash map, deep copy SEO keywords: Clone Graph, graph deep copy, DFS, BFS, LeetCode 133 Meta description: Deep-copy an undirected graph with a node-to-node map, explaining why memoization is mandatory and how DFS/BFS versions work, with runnable code in six languages. Target Readers LeetCode learners practicing graph traversal and deep-copy patterns Engineers who duplicate object graphs, workflow graphs, or topology graphs Developers who want one reusable template for “clone with cycles” Background / Motivation Many “copy” problems are actually identity-preservation problems. ...
Subtitle / Summary Level order traversal is the entry point of the binary-tree BFS template. The real key is not merely “use a queue”, but “separate one level from the next correctly”. This ACERS guide explains the level-size pattern, the DFS depth-bucket alternative, and engineering situations where grouped-by-depth traversal is useful. Reading time: 10-12 min Tags: Hot100, binary tree, BFS, DFS, queue, level order traversal SEO keywords: Hot100, Binary Tree Level Order Traversal, BFS, queue, level order traversal, LeetCode 102 Meta description: A systematic guide to LeetCode 102 from level-by-level BFS to DFS depth buckets, with engineering scenarios and runnable multi-language implementations. Target Readers Hot100 learners who want to make the BFS tree template stable Developers who can traverse a tree but still mix up current-level and next-level boundaries Engineers who need to group tree-shaped data by depth for display or execution Background / Motivation LeetCode 102 is one of the most standard tree-BFS starter problems. ...
Subtitle / Summary The hard part of Symmetric Tree is not traversal itself, but comparison direction. You are not comparing left to left and right to right. You are comparing mirrored positions. This ACERS guide explains the mirror-recursion contract, the BFS queue-of-pairs variant, and real engineering cases where symmetry checking matters. Reading time: 10-12 min Tags: Hot100, binary tree, DFS, BFS, symmetry SEO keywords: Hot100, Symmetric Tree, mirror recursion, binary tree symmetry, BFS, LeetCode 101 Meta description: A systematic guide to LeetCode 101 from mirror recursion to pairwise BFS symmetry checks, with engineering scenarios and runnable multi-language implementations. Target Readers Hot100 learners moving from Same Tree to mirror comparison Developers who can write ordinary tree recursion but still mix up outside and inside pairs Engineers who need left-right symmetry validation for layouts, topology templates, or mirrored structures Background / Motivation LeetCode 101 is excellent training for directional thinking in tree problems: ...
Subtitle / Summary The real challenge in LeetCode 100 is not “can you traverse a tree”, but “can you compare two trees node by node in lockstep”. This ACERS guide explains the synchronous-recursion contract, the queue-of-pairs BFS variant, and why the pattern matters in real engineering work. Reading time: 9-11 min Tags: Hot100, binary tree, DFS, BFS, tree comparison SEO keywords: Hot100, Same Tree, binary tree comparison, synchronous recursion, BFS, LeetCode 100 Meta description: A systematic guide to LeetCode 100 from synchronous recursion to pairwise BFS comparison, with engineering scenarios and runnable multi-language implementations. Target Readers Hot100 learners who want to build a stable “compare two trees together” template Developers who can write DFS on one tree but get confused once two trees must be checked in parallel Engineers who need structural-equivalence checks for config trees, component trees, or syntax trees Background / Motivation When many people first see LeetCode 100, the instinct is: ...
Subtitle / Summary Invert Binary Tree looks tiny, but it is one of the fastest ways to test whether you really understand recursive structure on trees. This guide uses LeetCode 226 to break down the essence of “swap left and right subtrees”, covers both recursion and BFS, and shows how the same idea transfers to engineering scenarios. Reading time: 8-10 min Tags: Hot100, binary tree, recursion, BFS, tree transformation SEO keywords: Hot100, Invert Binary Tree, tree mirror, recursion, BFS, LeetCode 226 Meta description: Learn the recursive and BFS solutions for LeetCode 226, then extend the idea to layout mirroring and structural transformations. Target Readers Hot100 learners who want to verify whether they truly understand “apply recursion to every node in the whole tree” Developers who instinctively start traversing any tree problem, but are unsure when to process the current node Engineers who need tree mirroring, layout inversion, or symmetric structural transforms Background / Motivation The code for LeetCode 226 is usually very short, but the thinking pattern is extremely typical: ...