POJ 3281 Dining（最大流）

http://poj.org/problem?id=3281

题意：

有n头牛，F种食物和D种饮料，每头牛都有自己喜欢的食物和饮料，每种食物和饮料只能给一头牛，每头牛需要1食物和1饮料。问最多能满足几头牛的需求。

思路：

 #include<iostream>
 #include<algorithm>
 #include<cstring>
 #include<cstdio>
 #include<sstream>
 #include<vector>
 #include<stack>
 #include<queue>
 #include<cmath>
 #include<map>
 #include<set>
 using namespace std;
 typedef long long ll;
 typedef long long ull;
 typedef pair<int,int> pll;
 const int INF = 0x3f3f3f3f;
 const int maxn =  + ;

 struct Edge
 {
     int from,to,cap,flow;
     Edge(int u,int v,int w,int f):from(u),to(v),cap(w),flow(f){}
 };

 struct Dinic
 {
     int n,m,s,t;
     vector<Edge> edges;
     vector<int> G[maxn];
     bool vis[maxn];
     int cur[maxn];
     int d[maxn];

     void init(int n)
     {
         this->n=n;
         for(int i=;i<n;++i) G[i].clear();
         edges.clear();
     }

     void AddEdge(int from,int to,int cap)
     {
         edges.push_back( Edge(from,to,cap,) );
         edges.push_back( Edge(to,from,,) );
         m=edges.size();
         G[from].push_back(m-);
         G[to].push_back(m-);
     }

     bool BFS()
     {
         queue<int> Q;
         memset(vis,,sizeof(vis));
         vis[s]=true;
         d[s]=;
         Q.push(s);
         while(!Q.empty())
         {
             int x=Q.front(); Q.pop();
             for(int i=;i<G[x].size();++i)
             {
                 Edge& e=edges[G[x][i]];
                 if(!vis[e.to] && e.cap>e.flow)
                 {
                     vis[e.to]=true;
                     d[e.to]=d[x]+;
                     Q.push(e.to);
                 }
             }
         }
         return vis[t];
     }

     int DFS(int x,int a)
     {
         if(x==t || a==) return a;
         int flow=, f;
         for(int &i=cur[x];i<G[x].size();++i)
         {
             Edge &e=edges[G[x][i]];
             if(d[e.to]==d[x]+ && (f=DFS(e.to,min(a,e.cap-e.flow) ) )>)
             {
                 e.flow +=f;
                 edges[G[x][i]^].flow -=f;
                 flow +=f;
                 a -=f;
                 if(a==) break;
             }
         }
         return flow;
     }

     int Maxflow(int s,int t)
     {
         this->s=s; this->t=t;
         int flow=;
         while(BFS())
         {
             memset(cur,,sizeof(cur));
             flow +=DFS(s,INF);
         }
         return flow;
     }
 }DC;

 int n,F,D;

 int main()
 {
     //freopen("in.txt","r",stdin);
     while(~scanf("%d%d%d",&n,&F,&D))
     {
         int src=, dst=F+D+*n+;
         DC.init(dst+);

         for(int i=;i<=F;i++)  DC.AddEdge(src,i,);
         for(int i=;i<=D;i++)  DC.AddEdge(F+i,dst,);

         for(int i=;i<=n;i++)
         {
             DC.AddEdge(F+D+i,F+D+n+i,);
             int num1,num2;

             scanf("%d%d",&num1,&num2);
             while(num1--)
             {
                 int x;
                 scanf("%d",&x);
                 DC.AddEdge(x,F+D+i,);
             }
             while(num2--)
             {
                 int x;
                 scanf("%d",&x);
                 DC.AddEdge(F+D+n+i,F+x,);
             }
         }
         printf("%d\n",DC.Maxflow(src,dst));
     }
     return ;
 }