网络流

求最大流后判断下那些边的容量为0即可。。

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<iostream>
#include<vector>
#include<string>
#include<math.h>
#include<map>
#include<set>
#include<algorithm>
using namespace std;
#define MAXN 2000000
struct Edge
{
  int v, next, val;
  Edge( ) { }
  Edge( int V, int N, int Val): v(V), next(N), val(Val) {}             
}edge[MAXN];

int head[MAXN],size, flow, visit, sum, N, M, ans;
int gap[MAXN], h[MAXN];
int path[2000010],vsize;
int answer[2000010];
int S,T, vs;


void init( )
{
  for( int i = 0; i < MAXN; i++)
  {
      head[i] = -1;        
       
  }     
  size = 0;
  vsize = 0;
     
}


void AddEdge( int u, int v, int val)
{
   edge[size] = Edge(v, head[u], val);
   head[u]  = size++;
   
   edge[size] = Edge(u, head[v], val);
   head[v] = size++;
     
}


int dfs( int x, int flow = 0x7f7f7f7f)
{
   if( x == T )
   {
      return flow;    
   }
   int v, u, high = N, f, tf = 0;
   for( v = head[x]; v != -1; v = edge[v].next )
   {
      u = edge[v].v;
      if( edge[v].val > 0 )
      {
        if( h[x] == h[u] + 1 && (f = dfs(u, min(flow - tf, edge[v].val))))
        { 
          edge[v].val -= f;
          edge[v^1].val += f;
          path[u] = x;
          tf += f;
          if( h[S] >= N || tf == flow)
            return tf;       
        }
        high = min(high, h[u]);
      }   
        
   }
   if( tf  == 0 )
   {
      gap[h[x]]--;
      if( gap[h[x]] == 0 )
         h[S] = N;
      h[x] = high + 1; 
      gap[h[x]]++;     
   }
   return tf;    
}

void solve( )
{
   gap[0] = N;
   memset(h, 0, sizeof(h));
   sum = 0;
   while( h[S] < N )
   {
      sum += dfs( S );
   }
   vsize = 0;
   for( int i = 0; i < size; i++)
   {
     int val = edge[i].val;
     if( val == 0 )
     {
        path[vsize++] = i / 2 + 1;
        
     }   
    
   }
   vsize = unique(path, path + vsize) - path;
   printf("%d\n",vsize);
   for( int i = 0; i < vsize; i++)
   {   
       printf("%d\n",path[i]);
   }
   
}


int main( )
{
   int a, b, c;
   ans = 0;
   while( scanf("%d%d%d%d",&N,&M,&S,&T) != EOF )
   {
     init( );
     memset(answer,0,sizeof(answer));
     for( int i = 1; i <= M; i++)
     {
       scanf("%d%d%d",&a, &b, &c);
       AddEdge(a, b, c);
     
     }
     solve( );
   } 
   return 0; 
}