CodeForces - 939A，解题报告

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();
     }

 }