topic:
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
inputCopy
5
2 4 5 1 3
output
YES
inputCopy
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.
Title:
First enter an n, and then there are n numbers in the array, indicating who you like, such as a[1]=2; meaning that number 1 likes number 2. Ask if there is a love triangle, output YES, otherwise output NO
Ideas:
Determine whether the first person liked the person who liked him or not.
Code:
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <iostream>
#include <cmath>
using namespace std;
const int N=1e5+10;
int a[N];
int main()
{
ios::sync_with_stdio(false);
int n;
while(cin>>n)
{
memset(a,0,sizeof(a));
for(int i=1;i<=n;i++)
{
cin>>a[i];
}
int flag=0;
for(int i=1;i<=n;i++)
{
int x,y;
x=a[i];//自己喜欢的人
y=a[x];//自己喜欢的人喜欢的人
if(a[y]==i&&x!=i&&x!=y&&i!=y)
//自己喜欢的人不能是自己 自己喜欢的人不能喜欢自己 自己喜欢的人的喜欢的人不能是自己
flag=1;
if(flag==1)
break;
}
if(flag)
cout<<"YES"<<endl;
else
cout<<"NO"<<endl;
}
return 0;
}