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=220;
const int INF=1<<28;
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,-1,sizeof(pre));
    pre[s] = 0;
    int k;
    q.push(s);
    while(!q.empty())
    {
        k = q.front();
        q.pop();
        for (int i = 0; i <= n+1; i ++)
        {
            if (pre[i]==-1 && map[k][i] > 0)
            {
                pre[i] = k;
                if (i==d)
                    return 1;
                q.push(i);
            }
        }
    }
    return 0;
}
int maxflow(int s,int d)
{
    int maxf = 0;
    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[32];
    while(~scanf("%d%d%d%d",&n,&np,&nc,&m))
    {
        memset(map,0,sizeof(map));
        s = n;
        d = n+1;
        for (int i = 0; i < m; i ++)
        {
            scanf("%s",str);
            sscanf(str,"(%d,%d)%d",&u,&v,&w);
            map[u][v] = w;

        }
        for (int i = 0; i < np; i ++)
        {
            scanf("%s",str);
            sscanf(str,"(%d)%d",&v,&w);
            map[s][v] = w;
        }
        for (int i = 0; i < nc; i ++)
        {
            scanf("%s",str);
            sscanf(str,"(%d)%d",&u,&w);
            map[u][d] = w;
        }
        printf("%d
",maxflow(s,d));
    }
    return  0;
}