HDU-1596-find the safest road
http://acm.hdu.edu.cn/showproblem.php?pid=1596
SPFA算法
#include<iostream>
#include<cstdio>
#include<cstring>
using namespace std;
int n,m,st,ed,front,rear;
double mat[1005][1005],dis[1005];
int v[1005],q[1000001];
void spfa()
{
int i,p;
while(front<rear)
{
p=q[front++];
v[p]=0;
for(i=0;i<n;i++)
{
if(mat[p][i]==0||p==i)
continue;
if(dis[p]*mat[p][i]>dis[i])
{
dis[i]=dis[p]*mat[p][i];
if(!v[i])
{
q[rear++]=i;
v[i]=1;
}
}
}
}
}
int main()
{
int i,j;
while(scanf("%d",&n)!=EOF)
{
for(i=0;i<n;i++)
for(j=0;j<n;j++)
scanf("%lf",&mat[i][j]);
scanf("%d",&m);
while(m--)
{
scanf("%d%d",&st,&ed);
memset(v,0,sizeof(v));
memset(dis,0,sizeof(dis));
v[st-1]=1;
dis[st-1]=1;
front=0;
rear=1;
q[front]=st-1;
spfa();
if(dis[ed-1]!=0)
printf("%.3lf\n",dis[ed-1]);
else
printf("What a pity!\n");
}
}
return 0;
}