Christmas Spruce
题意翻译
### 题目描述
给定一棵树，其中的每个非叶子节点的儿子中，必须有不少于三个节点是叶子节点，满足输出 `YES`，否则输出 `NO`。
### 输入格式
第一行包含一个整数 $n(3\le n\le 1000)$，表示树的节点数。 接下来的 $n-1$ 行每行包含一个整数 $p_i(1\le p_i\le i)$，$p_i$ 表示第 $i+1$ 号节点的父亲。
题目描述
Consider a rooted tree. A rooted tree has one special vertex called the root. All edges are directed from the root. Vertex $ u $ is called a child of vertex $ v $ and vertex $ v $ is called a parent of vertex $ u $ if there exists a directed edge from $ v $ to $ u $ . A vertex is called a leaf if it doesn't have children and has a parent.
Let's call a rooted tree a spruce if its every non-leaf vertex has at least $ 3 $ leaf children. You are given a rooted tree, check whether it's a spruce.
The definition of a rooted tree can be found [here](https://goo.gl/1dqvzz).
输入输出格式
输入格式
The first line contains one integer $ n $ — the number of vertices in the tree ( $ 3<=n<=1000 $ ). Each of the next $ n-1 $ lines contains one integer $ p_{i} $ ( $ 1<=i<=n-1 $ ) — the index of the parent of the $ i+1 $ -th vertex ( $ 1<=p_{i}<=i $ ).
Vertex $ 1 $ is the root. It's guaranteed that the root has at least $ 2 $ children.
输出格式
Print "Yes" if the tree is a spruce and "No" otherwise.
输入输出样例
输入样例 #1
4
1
1
1
输出样例 #1
Yes
输入样例 #2
7
1
1
1
2
2
2
输出样例 #2
No
输入样例 #3
8
1
1
1
1
3
3
3
输出样例 #3
Yes
说明
The first example:
The second example:
It is not a spruce, because the non-leaf vertex $ 1 $ has only $ 2 $ leaf children.
The third example:
