使用并查集判断连通图

import java.util.Scanner;

public class Main {
    static int[] f = new int[1005];

    public static void init() {
        for (int i = 0; i < f.length; i++) {
            f[i] = i;
        }
    }

    public static int find(int x) {
        if (f[x] == x)
            return x;
        return f[x] = find(f[x]);
    }

    public static void union(int x, int y) {
        int fx = find(x);
        int fy = find(y);
        if (fx != fy) {
            f[fx] = fy;
        }
    }

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        while (sc.hasNext()) {
            init();
            int n = sc.nextInt();
            int m = sc.nextInt();
            for (int i = 0; i < m; i++) {
                union(sc.nextInt(), sc.nextInt());
            }
            int cnt = 0;
            for (int i = 1; i <= n; i++) {
                if (f[i] == i) {
                    cnt++;
                }
            }
            if (cnt == 1) {
                System.out.println("YES");
            } else {
                System.out.println("NO");
            }
        }
    }
}