Power Network(最大流（EK算法）)

http://poj.org/problem?id=1459

题意：有一个电路网络，每个节点可以产生、传递、消耗若干电量，有点线连接结点，每个电线有最大传输量，求这个网络的最大消费量。

思路：从源点到发电站连边，流量为发电量，从用户到汇点连边，流量为消费量，再根据电线连双向边，求最大流即可。

 #include<stdio.h>
 #include<string.h>
 #include<queue>
 const int N=;
 const int INF=<<;
 using namespace std;
 int map[N][N];
 int pre[N];
 int n,np,nc,m;

 int bfs(int s,int d)
 {
     queue<int>q;
     memset(pre,-,sizeof(pre));
     pre[s] = ;
     int k;
     q.push(s);
     while(!q.empty())
     {
         k = q.front();
         q.pop();
         for (int i = ; i <= n+; i ++)
         {
             if (pre[i]==- && map[k][i] > )
             {
                 pre[i] = k;
                 if (i==d)
                     return ;
                 q.push(i);
             }
         }
     }
     return ;
 }
 int maxflow(int s,int d)
 {
     int maxf = ;
     while(bfs(s,d))
     {
         int minf = INF;
         for (int i = d; i!=s; i = pre[i])
             //minf = minf < map[pre[i]][i] ? minf:map[pre[i]][i];
             minf = min(minf,map[pre[i]][i]);
         for (int i = d; i!=s; i = pre[i])
         {
             map[pre[i]][i] -= minf;
             map[i][pre[i]] += minf;
         }
         maxf += minf;
     }
     return maxf;
 }
 int main()
 {
     int s,d;
     int u,v,w;
     char str[];
     while(~scanf("%d%d%d%d",&n,&np,&nc,&m))
     {
         memset(map,,sizeof(map));
         s = n;
         d = n+;
         for (int i = ; i < m; i ++)
         {
             scanf("%s",str);
             sscanf(str,"(%d,%d)%d",&u,&v,&w);
             map[u][v] = w;

         }
         for (int i = ; i < np; i ++)
         {
             scanf("%s",str);
             sscanf(str,"(%d)%d",&v,&w);
             map[s][v] = w;
         }
         for (int i = ; i < nc; i ++)
         {
             scanf("%s",str);
             sscanf(str,"(%d)%d",&u,&w);
             map[u][d] = w;
         }
         printf("%d\n",maxflow(s,d));
     }
     return  ;
 }