Union Find

Subtitle / Summary Do not start this problem by memorizing a Union-Find template. Start from the pressure: each land cell looks like one island at first, but adjacent land cells must collapse into one connected component. Reading time: 10-12 min Tags: Hot100, Union-Find, DSU, matrix, connected components SEO keywords: LeetCode 200, Number of Islands, Union-Find, DSU, connected components Meta description: A pressure-first Python guide to LeetCode 200 that derives land-only count, 2D-to-1D ids, Union-Find merges, and the final runnable solution. Problem Requirement You are given an m x n 2D character grid: ...

Subtitle / Summary The key signal in this problem is not a component count. It is a failed union: if two endpoints are already connected, adding the current edge closes a cycle. Reading time: 8-10 min Tags: Union-Find, DSU, graph, tree, cycle detection SEO keywords: LeetCode 684, Redundant Connection, Union-Find, DSU, cycle detection Meta description: A pressure-first Python guide to LeetCode 684 that derives 1-indexed Union-Find, bool-returning union, ordered edge scanning, and final checks. Problem Requirement You are given an undirected graph. It started as a tree with n nodes labeled from 1 to n, then one extra edge was added. ...

Subtitle / Summary Union-Find is not a pair of function names to memorize first. It solves one concrete problem: decide whether two nodes already belong to the same set, and merge two sets when a connection appears. Reading time: 10-12 min Tags: Hot100, Union-Find, DSU, graph, connectivity SEO keywords: Union-Find, DSU, Disjoint Set Union, find, union, count, path compression Meta description: A Python Union-Find template guide that derives parent, find, union, count, connected, path compression, and connected component counting. A - Algorithm: Start From Set-Merging Pressure Tiny task: merge sets and answer connectivity Suppose we have 5 nodes: ...

Problem Requirement You are given an n x n matrix isConnected, where there are n cities. If isConnected[i][j] == 1, city i and city j are directly connected. If two cities can reach each other through one or more directly connected cities, they belong to the same province. The problem asks you to return the total number of provinces. The easiest mistake is to think that the answer is the number of 1s in the matrix, or only the number of directly connected pairs. That is not what the problem asks. ...