SPFA ----模板 O(kE) (k一般不超过2)

原理：若一个点入队的次数超过顶点数V，则存在负环；

 #include "bits/stdc++.h"

 using namespace std;
 const int maxN =  ;
 struct Edge
 {
     int    to , next , w ;
 } e[ maxN ];

 int    n,m,cnt,p[ maxN ],Dis[ maxN ];
 int    In[maxN ];
 bool    visited[ maxN ];

 void    Add_Edge ( const int x , const int y , const int z )
 {
     e[ ++cnt ] . to = y ;
     e[ cnt ] . next = p[ x ];
     e[ cnt ] . w = z ;
     p[ x ] = cnt ;
     return ;
 }

 bool    Spfa(const int S)
 {
     int    i,t,temp;
     queue<int>    Q;
     memset ( visited ,  , sizeof ( visited ) ) ;
     memset ( Dis , 0x3f , sizeof ( Dis ) ) ;
     memset ( In ,  , sizeof ( In ) ) ;

     Q.push ( S ) ;
     visited [ S ] = true ;
     Dis [ S ] =  ;

     while( !Q.empty ( ) )
     {
         t = Q.front ( ) ;Q.pop ( ) ;visited [ t ] = false ;
         for( i=p[t] ; i ; i = e[ i ].next )
         {
             temp = e[ i ].to ;
             if( Dis[ temp ] > Dis[ t ] + e[ i ].w )
             {
                 Dis[ temp ] =Dis[ t ] + e[ i ].w ;
                 if( !visited[ temp ] )
                 {
                     Q.push(temp);
                     visited[temp]=true;
                     if(++In[temp]>n)return false;
                 }
             }
         }
     }
     return true;
 }

 int main ( )
 {
     int    S , T ;

     scanf ( "%d%d%d%d" , &n , &m , &S , &T ) ;
     for(int i= ; i<=m ; ++i )
     {
         int x , y , _ ;
         scanf ( "%d%d%d" , &x , &y , &_ ) ;
         Add_Edge ( x , y , _  ) ;
     }

     if ( !Spfa ( S ) ) printf ( "FAIL!\n" ) ;
     else               printf ( "%d\n" , Dis[ T ] ) ;

     return ;
 }