CF 939ALove Triangle

A. Love Triangle

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.

Input

The 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.

Output

Output «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 Note

In first example plane 2 likes plane 4, plane 4 likes plane 1, plane 1 likes plane 2 and that is a love triangle.

In second example there are no love triangles.

题目大意：

给出一个数组，找出里面是否有三个数满足以其中一个数为下标（从1开始）的数等于

另一个数且这另一个数等于那个数的下标。

代码：

#include<iostream> #include<algorithm> #include<cstdio> int a[5001]; using namespace std; int main() { int n; cin>>n; for(int i=1;i<=n;i++) { scanf("%d",&a[i]); } for(int i=1;i<=n;i++) { if(a[a[a[i]]]==i) { cout<<"YES"<<endl; return 0; } } cout<<"NO"<<endl; return 0; }