题目:http://www.lydsy.com/JudgeOnline/problem.php?id=2561
没什么好说的,两遍最小割完事。。。
代码:
#include <cstdio>
#include <algorithm>
#include <cstring>
using namespace std ;
const int maxn = 20100 , inf = 0x7fffffff , maxm = 200010 ;
struct network {
struct edge {
edge *next , *pair ;
int t , f ;
} *head[ maxn ] ;
void Init( ) {
memset( head , 0 , sizeof( head ) ) ;
}
void Add( int s , int t , int f ) {
edge *p = new( edge ) ;
p -> t = t , p -> f = f , p -> next = head[ s ] ;
head[ s ] = p ;
}
void AddEdge( int s , int t , int f ) {
Add( s , t , f ) , Add( t , s , f ) ;
head[ s ] -> pair = head[ t ] , head[ t ] -> pair = head[ s ] ;
}
int gap[ maxn ] , h[ maxn ] , S , T , V ;
edge *d[ maxn ] ;
int sap( int v , int flow ) {
if ( v == T ) return flow ;
int rec = 0 ;
for ( edge *p = d[ v ] ; p ; p = p -> next ) if ( p -> f && h[ v ] == h[ p -> t ] + 1 ) {
int ret = sap( p -> t , min( flow - rec , p -> f ) ) ;
p -> f -= ret , p -> pair -> f += ret , d[ v ] = p ;
if ( ( rec += ret ) == flow ) return flow ;
}
if ( ! ( -- gap[ h[ v ] ] ) ) h[ S ] = V ;
gap[ ++ h[ v ] ] ++ , d[ v ] = head[ v ] ;
return rec ;
}
int maxflow( ) {
int flow = 0 ;
for ( int i = 0 ; i ++ < V ; ) {
h[ i ] = gap[ i ] = 0 , d[ i ] = head[ i ] ;
}
gap[ 0 ] = V ;
for ( ; h[ S ] < V ; flow += sap( S , inf ) ) ;
return flow ;
}
} net ;
int n , m , E[ maxm ][ 3 ] , u , v , l , ans ;
int main( ) {
scanf( "%d%d" , &n , &m ) ;
for ( int i = 0 ; i ++ < m ; ) scanf( "%d%d%d" , &E[ i ][ 0 ] , &E[ i ][ 1 ] , &E[ i ][ 2 ] ) ;
scanf( "%d%d%d" , &u , &v , &l ) ;
net.Init( ) ;
net.S = u , net.T = v , net.V = n ;
for ( int i = 0 ; i ++ < m ; ) if ( E[ i ][ 2 ] < l ) {
net.AddEdge( E[ i ][ 0 ] , E[ i ][ 1 ] , 1 ) ;
}
ans = net.maxflow( ) ;
net.Init( ) ;
for ( int i = 0 ; i ++ < m ; ) if ( E[ i ][ 2 ] > l ) {
net.AddEdge( E[ i ][ 0 ] , E[ i ][ 1 ] , 1 ) ;
}
printf( "%d\n" , ans += net.maxflow( ) ) ;
return 0 ;
}