Codeforces Round #464 (Div. 2) A. Love Triangle[判断是否存在三角恋]
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.
[题意]:判断是否存在A喜欢B,B喜欢C,C喜欢A.(即三角恋)
[分析]:a[a[a[i]]]==i可以判断存在三角恋
[代码]:
![](https://images.cnblogs.com/OutliningIndicators/ContractedBlock.gif)
#include<bits/stdc++.h> using namespace std; const int maxn = 5000+10; int main() { int n,a[maxn],ans,j,k; while(cin>>n) { ans=0; for(int i=1;i<=n;i++) { cin>>a[i]; } for(int i=1;i<=n;i++) { j=a[i]; k=a[j]; if(a[k]==i && i!=j && j!=k && i!=k) ans++; } printf("%s\n",ans?"YES":"NO"); } }
![](https://images.cnblogs.com/OutliningIndicators/ContractedBlock.gif)
#include<bits/stdc++.h> using namespace std; const int maxn = 5000+10; int main() { int n,a[maxn],f,ans; while(cin>>n) { for(int i=1;i<=n;i++) cin>>a[i]; f=0; for(int i=1;i<=n;i++) { if(a[a[a[i]]]==i) f=1; } printf("%s\n",f?"YES":"NO"); } }