题目

代码

#include <bits/stdc++.h>
using namespace std;
const int N = 1e5+10;
const int M = 2e5+10;
int h[N], e[M], ne[M], idx, w[M];
double p[M];
int cnt[N];
double E;
int n, m;
void add(int a, int b, int c)  // 添加一条边a->b
{
    p[idx] = 1, w[idx] = c, e[idx] = b, ne[idx] = h[a], h[a] = idx ++;
}
void dfs(int f, int u, int l, double P)
{
    for(int k = h[u]; ~k; k = ne[k])
    {
        int j = e[k];
        if(j == f) continue;
        dfs(u, j, l+w[k], P*p[k]);
    }
    
    if(u == n) E += P * l;
}
int main()
{
    scanf("%d%d", &n, &m);
    
    memset(h, -1, sizeof h);
    for(int i = 1; i <= m; i++)
    {
        int a, b, c;
        scanf("%d%d%d", &a, &b, &c);
        add(a, b, c);
        cnt[a]++;
    }
    
    for(int i = 1; i <= n; i++)
        for(int k = h[i]; ~k; k = ne[k])
            p[k] /= cnt[i];
            
    
    dfs(-1, 1, 0, 1);
    printf("%.2f", E);
    return 0;
}