SPFA(G,w,s)
1. INITIALIZE-SINGLE-SOURCE(G,s)
2. INITIALIZE-QUEUE(Q)
3. ENQUEUE(Q,s)
4. While Not EMPTY(Q)
5. Do u<-DLQUEUE(Q)
6. For 每条边(u,v) in E[G]
7. Do tmp<-d[v]
8. Relax(u,v,w)
9. If (d[v] < tmp) and (v不在Q中)
10. ENQUEUE(Q,v)
#include<iostream>
using namespace std;
#define MAX 500
#define maxn 0x7fffffff
int q[3*MAX+1],d[3*MAX+1];
bool flag[3*MAX+1];
int n,from,to;
int spfa()//源点from 目标点to 
{   memset(flag,0,sizeof(flag));
    int i,j,rear,front,now;
    rear=front=0;
    q[rear++]=from;
    flag[from]=1;
    for(i=1;i<=n;i++)
        d[i]=maxn;
    d[from]=0;
    while(front<=rear)
    {    now=q[front++];
         flag[now]=0;
         for(i=1;i<=n;i++)
         {   if(f[now][i]&&d[now]+f[now][i]<d[i])
             {   d[i]=d[now]+f[now][i];
                 if(!flag[i])
                 {   flag[i]=1;
                     q[rear++]=i;
                 }
             }
         }
    }
    return d[to];
}