union-find

public class UF {

    // 存储一棵树
    private int[] parent;
    // 记录树的“重量”
    private int[] size;
    // 连通分量个数
    private int count;

    public UF(int n) {
        this.count = n;
        parent = new int[n];
        size = new int[n];
        for (int i = 0; i < n; i++) {
            parent[i] = i;
            size[i] = 1;
        }
    }

    public void union(int p, int q) {
        int rootP = find(p);
        int rootQ = find(q);
        if (rootP == rootQ) return;
        if (size[rootP] < size[rootQ]) {
            parent[rootP] = rootQ;
            size[rootQ] += size[rootP];
        } else {
            parent[rootQ] = rootP;
            size[rootP] += size[rootQ];
        }
        count--;
    }

    public int find(int x) {
        while (x != parent[x]) {
            // 进行路径压缩
            parent[x] = parent[parent[x]];
            x = parent[x];
        }
        return x;
    }

    public boolean connected(int p, int q) {
        return find(p) == find(q);
    }

    public int count() {
        return this.count;
    }
    
}