cf 464 A. Love Triangle

A. Love Triangle time limit per test 1 second memory limit per test 256 megabytes input standard input output standard output

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; }未来的我一定会感谢现在正在成长的我