As you could know there are no male planes nor female planes. However, each plane on Earth likes some other plane. There are n planes on Earth, numbered from 1 to n, and the plane with number i likes the plane with number fi, where 1 ≤ fi ≤ n and fi ≠ i.
We call a love triangle a situation in which plane A likes plane B, plane B likes plane C and plane C likes plane A. Find out if there is any love triangle on Earth.
InputThe first line contains a single integer n (2 ≤ n ≤ 5000) — the number of planes.
The second line contains n integers f1, f2, ..., fn (1 ≤ fi ≤ n, fi ≠ i), meaning that the i-th plane likes the fi-th.
OutputOutput «YES» if there is a love triangle consisting of planes on Earth. Otherwise, output «NO».
You can output any letter in lower case or in upper case.
Examples input Copy 5 2 4 5 1 3 output YES input Copy 5 5 5 5 5 1 output NO题意：给你n个数，第i个数代表：第i个人喜欢第a[i]个人，问你是否存在三角恋。
思路：第i个下标对应的值a[i]即为喜欢的人，所以很容易发现如果a[a[a[i]]]==i，即为三角恋。
#include<bits/stdc++.h> #define ll long long using namespace std; ll a[5050],n,flag; int main() { while(~scanf("%lld",&n)) { for(ll i=1;i<=n;i++)scanf("%lld",&a[i]); for(ll i=1;i<=n;i++) { if(a[a[a[i]]]==i)flag=1; //三角恋判定 } if(flag==1)printf("YES\n"); else printf("NO\n"); } return 0; }