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前言

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并查集（Union-Find）算法是一个专门针对「动态连通性」的算法，我之前写过两次，因为这个算法的考察频率高，而且它也是最小生成树算法的前置知识，所以我整合了本文，争取一篇文章把这个算法讲明白。

一、力扣323.无向图中连通分量的数目

class UF{private int[]parent;private int count;public UF(int n){parent = new int[n];count = n;for(int i = 0; i < n; i ++){parent[i] = i;}}public int getCount(){return count;}public int find(int x){while(parent[x] != x){parent[x] = find(parent[x]);}return parent[x];}public void union(int x, int y){int rootX = find(x);int rootY = find[y];if(rootX == rooY){return;}parent[rootX] = rootY;count --;}public boolean getConnection(int x, int y){int rootX = find(x);int rootY = find(y);return rootX == rootY;}public int fun(int n, int[][] edges){UF uf = new UF(n);for(int i = 0; i < n; i ++){union(edges[i][0], edges[i][1]);}return getCount();}
}

二、力扣130. 被围绕的区域

class Solution {public void solve(char[][] board) {int[][] arr = new int[][]{{0,1},{0,-1},{-1,0},{1,0}};if(board == null || board.length == 0){return;}int m = board.length;int n = board[0].length;int dummy = m * n;UF uf = new UF(dummy+1);for(int i = 0; i < m; i ++){if(board[i][0] == 'O'){uf.union(i*n,dummy);}if(board[i][n-1] == 'O'){uf.union(i*n+n-1,dummy);}}for(int i = 0; i < n; i ++){if(board[0][i] == 'O'){uf.union(i,dummy);}if(board[m-1][i] == 'O'){uf.union((m-1)*n+i,dummy);}}for(int i = 1; i < m-1; i ++){for(int j = 1; j < n-1; j ++){if(board[i][j] == 'O'){for(int k = 0; k < 4; k ++){int X = i + arr[k][0];int Y = j + arr[k][1];if(board[X][Y] == 'O'){uf.union(X*n+Y,i*n+j);}}}}}for(int i = 0; i < m; i ++){for(int j = 0; j < n; j ++){if(board[i][j] == 'O' && !uf.getConnection(i*n+j, dummy)){board[i][j] = 'X';}}}}class UF{private int count;private int[] parent;public UF(int n){count = n;parent = new int[n];for(int i = 0; i < n; i ++){parent[i] = i;}}public int getCount(){return count;}public int find(int x){if(parent[x] != x){parent[x] = find(parent[x]);}return parent[x];}public void union(int x, int y){int rootX = find(x);int rootY = find(y);if(rootX == rootY){return;}parent[rootX] = rootY;count --;}public boolean getConnection(int x, int y){int rootX = find(x);int rootY = find(y);return rootX == rootY;}}
}