CodeForces

题意：给出一个n个点有向图，问是否存在三个点，这三个点构成一个回路。n<=5000

模拟即可。

注意是必须三个点 多了居然不行。

import java.util.*;
public class Main {

    public static final int maxn = 5000;
    public static int [] v = new int [maxn+10];
    public static int [] f = new int [maxn+10];
    
    public static int DFS(int x, int t) {
        if (v[x]>0) return t - v[x];
        v[x] = t;
        int k = DFS(f[x], t+1);
        v[x] = 0;
        return k;
    }
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        int n = input.nextInt();
        
        
        for (int i = 1; i<=n; i++) f[i] = input.nextInt();
        boolean flag = true;
        for (int i = 1; i<=n; i++) {
            if (3 == DFS(i, 1)) {
                System.out.println("YES");
                flag = false;
                break;
            }
        }
        if(flag) System.out.println("NO");
        input.close();
    }

}