Drainage Ditches(网络流(EK算法))

计算最大流，EK算法模板题。

 #include <stdio.h>
 #include <string.h>
 #include <queue>
 using namespace std;
 const int maxn=;
 const int maxm=;
 const int INF=<<;
 int n,m,s,t,cnt,max_flow;
 int head[maxn],pre[maxn],a[maxn];

 struct node
 {
     int u;
     int v;
     int cap;
     int next;
 } edge[maxm];
 void init()
 {
     memset(head,-,sizeof(head));
     cnt = ;
 }
 void add(int u,int v,int cap)//用邻接表存储
 {
     edge[cnt].u = u;
     edge[cnt].v = v;
     edge[cnt].cap = cap;
     edge[cnt].next = head[u];
     head[u] = cnt++;
 }
 void EK(int s)
 {
     queue<int>q;
     max_flow = ;
     for (;;)
     {
         memset(a,,sizeof(a));
         q.push(s);
         a[s] = INF;//源点容量为无穷
         while(!q.empty())
         {
             int u = q.front();
             q.pop();
             for (int i = head[u]; i!=-; i=edge[i].next)
             {
                 int v = edge[i].v;
                 if (!a[v] && edge[i].cap > )
                 {
                     q.push(v);
                     pre[v] = i;// 记录V的前驱的下标
                     a[v] = std::min(a[u],edge[i].cap);//更新每个节点的flow
                 }
             }
         }
         if(a[t]==)
             break;
         int i;
         for (int u = t; u!=s; u = edge[i].u)//修改增光路
         {
             i = pre[u];
             edge[i].cap -= a[t];//正向的边容量减去残余量
             edge[i^].cap += a[t];//反向的边容量(初始为0)加上残余量
         }
         max_flow += a[t];
     }
 }
 int main()
 {
     while(~scanf("%d%d",&m,&n))
     {
         init();
         while(m--)
         {
             int u,v,w;
             scanf("%d%d%d",&u,&v,&w);
             add(u,v,w);
             add(v,u,);//加反向边

         }
         s = ;
         t = n;
         EK(s);
         printf("%d\n",max_flow);
     }
     return ;
 }