并查集模板

class Solution {
    public int numIslands(char[][] grid) {
        int rows = grid.length;
        int cols = grid[0].length;
        UnionFind unionFind = new UnionFind(0, new int[rows * cols]);
        for (int i = 0; i < rows; i ++) {
            for (int j = 0; j < cols; j ++) {
                if (grid[i][j] == '1') {
                    unionFind.count ++;
                    unionFind.parent[i * cols + j] = i * cols + j;
                }
            }
        }

        for (int i = 0; i < rows; i ++) {
            for (int j = 0; j < cols; j ++) {
                if (grid[i][j] == '1') {
                    grid[i][j] = '0';

                    if (i - 1 >= 0 && grid[i - 1][j] == '1') {
                        unionFind.union(i * cols + j, (i - 1) * cols + j);
                    }

                    if (i + 1< rows && grid[i + 1][j] == '1') {
                        unionFind.union(i * cols + j, (i + 1) * cols + j);
                    }

                    if (j - 1 >= 0 && grid[i][j - 1] == '1') {
                        unionFind.union(i * cols + j, i * cols + j - 1);
                    }

                    if (j + 1 < cols && grid[i][j + 1] == '1') {
                        unionFind.union(i * cols + j, i * cols + j + 1);
                    }
                }
            }
        }
        return unionFind.count;

    }
}
class UnionFind {
    public int count;
    public int[] parent;

    public UnionFind(int count, int[] parent) {
        this.count = count;
        this.parent = parent;
    }

    public int find(int[] parent, int index) {
        if (parent[index] == index) {
            return index;
        }
        return find(parent, parent[index]);
    }

    public void union(int first, int second) {
        int firstRoot = find(parent, first);
        int secondRoot = find(parent, second);
        if (firstRoot != secondRoot) {
            count--;
            parent[firstRoot] = secondRoot;
        }
    }

}