cf 464 A. Love Triangle

最新推荐文章于 2021-05-02 20:53:10 发布

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本文介绍了一个关于判断是否存在飞机间的爱情三角的算法问题。输入为若干飞机及其喜欢的对象编号，输出是否存在爱情三角的情况。

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 f_i, where 1 ≤ f_i ≤ n and f_i ≠ 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, ..., f_n (1 ≤ f_i ≤ n, f_i ≠ i), meaning that the i-th plane likes the f_i-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.

#include <cstdio>
#include <iostream>
#include <cstring>
#include <algorithm>
#include <cmath>
#include <vector>
#include <queue>
using namespace std;
#define INF 0x7f
const int maxn =10005; 
typedef long long ll ;
#define f(i,l,r) for(int i=l;i<=r;++i)
#define g(i,l,r) for(int i=l;i>=r;--i)
int n ;
int a[maxn];
bool flag=false;
int main()
{
	cin>>n;
	f(i,1,n)
		cin>>a[i];
	f(i,1,n)
	{
		if(a[a[a[i]]]==i)
			flag=true;
	}
	 if(flag)cout<<"YES"<<endl;
	 else cout<<"NO"<<endl;

	return 0;

}