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；

 1 #include "bits/stdc++.h"
 2
 3 using namespace std;
 4 const int maxN = 200010 ;
 5 struct Edge
 6 {
 7     int    to , next , w ;
 8 } e[ maxN ];
 9
10 int    n,m,cnt,p[ maxN ],Dis[ maxN ];
11 int    In[maxN ];
12 bool    visited[ maxN ];
13
14 void    Add_Edge ( const int x , const int y , const int z )
15 {
16     e[ ++cnt ] . to = y ;
17     e[ cnt ] . next = p[ x ];
18     e[ cnt ] . w = z ;
19     p[ x ] = cnt ;
20     return ;
21 }
22
23 bool    Spfa(const int S)
24 {
25     int    i,t,temp;
26     queue<int>    Q;
27     memset ( visited , 0 , sizeof ( visited ) ) ;
28     memset ( Dis , 0x3f , sizeof ( Dis ) ) ;
29     memset ( In , 0 , sizeof ( In ) ) ;
30
31     Q.push ( S ) ;
32     visited [ S ] = true ;
33     Dis [ S ] = 0 ;
34
35     while( !Q.empty ( ) )
36     {
37         t = Q.front ( ) ;Q.pop ( ) ;visited [ t ] = false ;
38         for( i=p[t] ; i ; i = e[ i ].next )
39         {
40             temp = e[ i ].to ;
41             if( Dis[ temp ] > Dis[ t ] + e[ i ].w )
42             {
43                 Dis[ temp ] =Dis[ t ] + e[ i ].w ;
44                 if( !visited[ temp ] )
45                 {
46                     Q.push(temp);
47                     visited[temp]=true;
48                     if(++In[temp]>n)return false;
49                 }
50             }
51         }
52     }
53     return true;
54 }
55
56 int main ( )
57 {
58     int    S , T ;
59
60     scanf ( "%d%d%d%d" , &n , &m , &S , &T ) ;
61     for(int i=1 ; i<=m ; ++i )
62     {
63         int x , y , _ ;
64         scanf ( "%d%d%d" , &x , &y , &_ ) ;
65         Add_Edge ( x , y , _  ) ;
66     }
67
68     if ( !Spfa ( S ) ) printf ( "FAIL!\n" ) ;
69     else               printf ( "%d\n" , Dis[ T ] ) ;
70
71     return 0;
72 }