CodeForces 839C

起初误以为到每个叶子的概率一样于是....

/*
CodeForces 839C - Journey [ DFS,期望 ]	|	Codeforces Round #428 (Div. 2)
*/
#include <bits/stdc++.h>
using namespace std;
const int N = 100005;
int n;
vector<int> G[N];
double dp[N], val[N];
bool vis[N];
void dfs(int u, int pre)
{
    bool flag = 0;
    for (auto&v : G[u])
    {
        if (v == pre) continue;
        flag = 1;
        dp[v] = dp[u]+1;
        val[v] = val[u] / (G[u].size()-1);
        dfs(v, u);
    }
    if (!flag) vis[u] = 1;
}
int main()
{
    scanf("%d", &n);
    for (int i = 1; i < n; i++)
    {
        int u, v; scanf("%d%d", &u, &v);
        G[u].push_back(v);
        G[v].push_back(u);
    }
    G[1].push_back(1);
    val[1] = 1;
    dfs(1, 1);
    int cnt = 0;
    double ans = 0;
    for (int i = 1; i <= n; i++)
        if (vis[i]) ans += dp[i]*val[i];
    printf("%.15f
", ans);
}