绿豆蛙的归宿 期望dp+拓扑排序

#include <bits/stdc++.h>
using namespace std;
const int M = 200500;
struct Edge{
	int nxt,v;
	double val;
};
Edge edge[M];

double in[M],dg[M];
double f[M];
int n,m,cnt,head[M];
void add(int u,int v,double w){
	cnt++;
	edge[cnt].v = v;
	edge[cnt].val = w;
	edge[cnt].nxt = head[u];
	head[u] = cnt;
}
int read(){
	int x=0,dign=1;
	char c = getchar();
	while(c<'0'||c>'9'){
		if(c=='-')dign=-1;
		c = getchar();
	}
	while(c>='0'&&c<='9'){
		x = x*10+c-'0';
		c = getchar(); 
	}
	return x*dign;
} 
void spfa(){
	queue<int> q; 
	for(int i=1;i<=n;i++)if(!in[i])q.push(i);
	while(!q.empty()){
		int u = q.front(); q.pop();
		for(int v,i=head[u];i;i=edge[i].nxt){
			double w = edge[i].val;
			v = edge[i].v;
			f[v]+=(f[u]+w)/dg[v]; 
			in[v]--;
			if(!in[v])q.push(v);
		}
	}
}
int main()
{	
	n = read(); m = read();
	for(int i=0;i<m;i++){
		int u,v;double w;
		u = read(); v=read(); scanf("%lf",&w);
		add(v,u,w);//反向建图 
		in[u]++; dg[u]++;
	}
	spfa();
	printf("%.2lf",f[1]);
	return 0;
}