CF939A Love Triangle

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题目描述

飞机跟人一样，也会有三角恋。现在有$n$ 架飞机，编号从1~$n$ ,且第$i$ 号飞机喜欢$f_{i}$ 号飞机，且1<=$f_{i}$ <=$n$ ,$f_{i}$ ≠$i$ 。 当飞机$A$ 喜欢飞机$B$ ，且飞机$B$ 喜欢飞机$C$ ，且飞机$C$ 喜欢飞机$A$ ，我们称这种情况为三角恋。 现在请你找出是否存在三角恋。

输入格式

第一行一个数,飞机的个数$n$ ,且$1&lt;=n&lt;=5000$
第二行$n$ 个数，$f_{1},f_{2},...,f_{n}$ ,代表$i$ 号飞机喜欢$f_{i}$ 号飞机

输出格式

如果存在三角恋则输出"YES",否则输出“NO” 输出大小写都没问题

题目描述

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.

输入输出格式

输入格式：

The first line contains a single integer $n$ ( $2<=n<=5000$ ) — the number of planes.

The second line contains $n$ integers $f_{1},f_{2},...,f_{n}$ ( $1<=f_{i}<=n$ , $f_{i}≠i$ ), meaning that the $i$ -th plane likes the $f_{i}$ -th.

输出格式：

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.

输入输出样例

输入样例#1： 复制
5
2 4 5 1 3

输出样例#1： 复制
YES

输入样例#2： 复制
5
5 5 5 5 1

输出样例#2： 复制
NO


说明

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.

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