python/TangDou/AcWing/Math/QiWang/217_DaoTui.cpp

#include <bits/stdc++.h>
using namespace std;
const int N = 100010, M = N << 1;
int n, m;
int in[N], g[N]; //入度，入度的备份数组，原因：in在topsort中会不断变小受破坏

double f[N];

//链式前向星
int e[M], h[N], idx, w[M], ne[M];
void add(int a, int b, int c = 0) {
    e[idx] = b, ne[idx] = h[a], w[idx] = c, h[a] = idx++;
}

void topsort() {
    queue<int> q;
    q.push(n);
    f[n] = 0; // n到n的距离期望是0
    while (q.size()) {
        int u = q.front();
        q.pop();
        for (int i = h[u]; ~i; i = ne[i]) { //枚举每条入边(因为是反向图)
            int j = e[i];
            f[j] += (f[u] + w[i]) / g[j];
            in[j]--;
            if (in[j] == 0) q.push(j);
        }
    }
}
int main() {
    memset(h, -1, sizeof(h));
    cin >> n >> m;

    while (m--) {
        int a, b, c;
        cin >> a >> b >> c;
        add(b, a, c); //反向图,计算从n到1
        in[a]++;      //入度
        g[a] = in[a]; //入度数量
    }
    topsort();
    printf("%.2lf\n", f[1]);
    return 0;
}