学步园

AC code:

#include<iostream>
#include<stdio.h>
#include<queue>
#include<iomanip>
#include<string>
#include<cstring>
using namespace std;
#define N 50005
struct Edgenode
{
	int v,next;
	double cost;
}Edge[N*55];
int edgenumber,head[N];
int visited[N];
double Sum[N];
void SAFA(int s,int t,int total);
void addedge(int i,int v1,double value)
{
	Edge[edgenumber].v=v1;
	Edge[edgenumber].cost=value;
	Edge[edgenumber].next=head[i];
	head[i]=edgenumber++;
}

int main()
{
	int n,i,j,k,value,s,t,total;
	while(scanf("%d",&n)!=EOF)
	{
		memset(head,-1,sizeof(head));
		edgenumber=0;
		for(i=1;i<=n;i++)
		{
			cin>>j;
			while(j--)
			{
				cin>>k>>value;
				addedge(i,k,value/100.00);
			}
		}
		cin>>s>>t>>total;
		for(int q=0;q<=n;++q) { visited[q]=0; Sum[q]=total;}
		SAFA(s,t,total);
		if(Sum[t]>=total)
			cout<<"IMPOSSIBLE!"<<endl;
		else
			printf("%.2lf\n",Sum[t]);  
	}
	return 0;
}


void SAFA(int s,int t,int total)
{
	queue<int> q;
	q.push(s);
	double temp;
	int i,j,k;
	Sum[s]=0;
	visited[s]=true;
	while(!q.empty())
	{
		i=q.front();
		q.pop();
		for(j=head[i];j!=-1;j=Edge[j].next)
		{
			k=Edge[j].v;
			temp=Sum[i]+(total-Sum[i])*Edge[j].cost;
			if(Sum[k]<=temp) continue;
			Sum[k]=temp;
			if(visited[k]) continue;
			q.push(k);
			visited[k]=1;
		}
		visited[i]=0;
	}
}