洛谷 #4316. 绿豆蛙的归宿

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题意

有向图，等概率选择路径，问期望路径长

题解

概率Dp,f[i]表示i节点的期望路径长

因为Dp无后效性，所以反向建图，来一遍Toposort就好了

调试记录

没有反向建图

#include <cstdio>
#include <queue>
#define maxn 1000005
#define double long double

using namespace std;

struct node{
    int to, next, l;
}e[maxn];

int n, m, tot = 0, index[maxn], head[maxn];
double p[maxn], f[maxn];

void addedge(int u, int v, int l){
    e[++tot] = (node){v, head[u], l};
    head[u] = tot;
}

queue <int> q;
void Topo(){
    for (int i = 1; i <= n; i++) if (!index[i]) q.push(i);
    
    while (q.size()){
        int now = q.front(); q.pop();
        for (int i = head[now]; i; i = e[i].next){
            f[e[i].to] += (f[now] + e[i].l) / p[e[i].to];
            if (!--index[e[i].to]) q.push(e[i].to);
        }
    }
}

int main(){
    scanf("%d%d", &n, &m);
    
    for (int x, y, l, i = 1; i <= m; i++){
        scanf("%d%d%d", &x, &y, &l);
        addedge(y, x, l); index[x]++; p[x]++;
    }
    Topo();
    printf("%.2llf\n", f[1]);
    
    return 0;
}