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 = 200010 ;
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 , 0 , sizeof ( visited ) ) ; 
    memset ( Dis , 0x3f , sizeof ( Dis ) ) ; 
    memset ( In , 0 , sizeof ( In ) ) ;
    
    Q.push ( S ) ;
    visited [ S ] = true ;
    Dis [ S ] = 0 ;

    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=1 ; 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 0;
}