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.
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 «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.
5 2 4 5 1 3
YES
5 5 5 5 5 1
NO
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; }未來的我一定會感謝現在正在成長的我