Love Triangle

### 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 $1 \sim n$ 且第 $i$ 号飞机喜欢 $f_i$ 号飞机，且 $1 \le f_i \le n$，$f_i \ne i$。 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 $n$（$2 \le n \le 5000$）。 第二行 $n$ 个数，$f_1, f_2, \ldots , f_n$，表示 $i$ 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。

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.

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.

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.