CodeForces

先大喊：我是傻逼！

首先这个题很简单
n 个结点， n-1 条边， 不会出现环
我写 MLE 了，因为我很自信的在 dfs 里面判 v 的父结点的时候判成了第 1 个结点，所以，，GG

要是说求期望的话
推一下就能发现答案就是 到每个结点的概率之和

#include <iostream>#include <cstdio>#include <cstdlib>#include <cstring>#include <cmath>#include <set>#include <map>#include <stack>#include <queue>#include <ctype.h>#include <vector>#include <algorithm>#define in freopen("in.txt", "r", stdin)#define out freopen("out.txt", "w", stdout)// cout << "  ===  " << endl;using namespace std;typedef long long ll;const int maxn = 100000 + 7, INF = 0x7f7f7f7f, mod = 1e9+7;int n, vis[maxn];vector<int> a[maxn];double ans = 0;void init() {    int u, v;    for(int i = 0; i < n-1; ++i) {        scanf("%d%d", &u, &v);        a[u].push_back(v);        a[v].push_back(u);    }}void dfs(int x, double f) {    vis[x] = 1;    int m = a[x].size(), mm = a[x].size();    if(x != 1) m--;    if(m == 0) return ;    double t = (f * (1.0 / m));    for(int i = 0; i < mm; ++i) {        if(vis[a[x][i]]) continue;        ans += t;        dfs(a[x][i], t);    }    return;}int main() {    memset(vis, 0, sizeof vis);    scanf("%d", &n);    init();    dfs(1, 1.0);    printf("%.15lf", ans);    return 0;}