【模板】最大流之EdmondsKarp算法

他人博客详细讲解：http://www.cnblogs.com/zsboy/archive/2013/01/27/2878810.html

好像大概意思是不停地用bfs找一条增广链，并更新答案，直到找不到为止，构图时需构建反向弧，来让错误的路可以往回走。。

程序：

#include<iostream>
#include<cstdio>
#include<queue>
#include<cmath>
#include<vector>
#include<cstring>
using namespace std;
struct ding{
   int from,to,cap,now;
};
int a[20000],p[20000];
vector<ding>edge;
vector<int>g[20000];
int esize=0,n,m;
void add(int from1,int to1,int val)
{
  edge.push_back((ding){from1,to1,val,0});
  edge.push_back((ding){to1,from1,0,0});
  g[from1].push_back(esize);
  g[to1].push_back(esize+1);
  esize+=2;
}
int maxlow(int s,int t)
{
  int ans=0;
  while (true)
  {
      queue<int>q;
      q.push(s);
      memset(a,0,sizeof a);
      a[s]=2100000000;
      while (!q.empty())
      {
        int k=q.front(); q.pop();
      for (int i=0;i<=g[k].size()-1;i++)
      {
          ding enow=edge[g[k][i]];
          if ((a[enow.to]==0)&&(enow.now<enow.cap))
          {
            a[enow.to]=min(a[k],enow.cap-enow.now);
          p[enow.to]=g[k][i];
          q.push(enow.to);
        }
      }
      if (a[t]) break;
    }
    if (!a[t]) break;
    ans+=a[t];
    for (int i=t;i!=s;i=edge[p[i]].from)
    {
      edge[p[i]].now+=a[t];
      edge[p[i]^1].now-=a[t];
    }
  }
  return ans;
}
int main()
{
  int s,t,u,v,w;
  scanf("%d%d%d%d",&n,&m,&s,&t);
  for (int i=1;i<=m;i++)
  {
    scanf("%d%d%d",&u,&v,&w);
    add(u,v,w);
  }
  printf("%d\n",maxlow(s,t));
  return 0;
}