判断一颗树是否为云松树，其定义为非叶子的结点至少要有3个叶节点，从根结点往下跑即可，这里我用的队列维护。

#include <iostream>
#include <algorithm>
#include <cstdio>
#include <vector>
#include <list>
#include <stack>
#include <cstring>
#include <queue>
using namespace std;
#define MAX 100005
#define ll long long
#define INF 0x3f3f3f3f
#define mod 1000000007
int n;
vector<int> a[1005];
int main()
{
//  freopen("a.txt", "r", stdin);
//  freopen("b.txt", "w", stdout);
    int n, x;
    cin >> n;
    for (int i = 1; i < n; ++i) scanf("%d", &x), a[x].push_back(i + 1);
    bool f = true;
    queue<int> q;
    q.push(1);
    while (!q.empty()) {
        int t = q.front();
        q.pop();
        int c = a[t].size();
        if (!c) continue;
        for (int i = 0; i < a[t].size(); ++i) {
            int v = a[t][i];
            int si = a[v].size();
            if (si) c--;
            q.push(v);
        }
        if (c < 3) f = false;
    }
    if (f) cout << "Yes" << endl;
    else cout << "No" << endl;
    return 0;
}