BZOJ1693: [Usaco2007 Demo]Asteroids

n<=500 *n的格子，给m<=10000个格子有人，一炮可以清掉一行或一列的人（莫名的爽！）求最少几炮干掉所有人。

经典二分图模型！行成点，列成点，一个点就连接一行一列，表示这一行或这一列必选其一！

不喜欢匈牙利！跑网络流！

 #include<stdio.h>
 #include<string.h>
 #include<stdlib.h>
 #include<algorithm>
 #include<iostream>
 using namespace std;

 int n,m;
 #define maxn 1011
 #define maxm 22011
 struct Edge{int to,next,cap,flow;};
 const int inf=0x3f3f3f3f;
 struct Network
 {
     Edge edge[maxm];int first[maxn],le,n,s,t;
     void clear(int n)
     {
         memset(first,,sizeof(first));
         le=;this->n=n;
     }
     void in(int x,int y,int cap)
     {
         Edge &e=edge[le];
         e.to=y;e.cap=cap;e.flow=;
         e.next=first[x];first[x]=le++;
     }
     void insert(int x,int y,int cap) {in(x,y,cap);in(y,x,);}
     int dis[maxn],cur[maxn],que[maxn],head,tail;
     bool bfs()
     {
         memset(dis,,sizeof(dis));
         dis[s]=;
         que[head=(tail=)-]=s;
         while (head!=tail)
         {
             const int now=que[head++];
             for (int i=first[now];i;i=edge[i].next)
             {
                 Edge &e=edge[i];
                 if (e.cap>e.flow && !dis[e.to])
                 {
                     dis[e.to]=dis[now]+;
                     que[tail++]=e.to;
                 }
             }
         }
         return dis[t];
     }
     int dfs(int x,int a)
     {
         if (x==t || !a) return a;
         int flow=,f;
         for (int &i=cur[x];i;i=edge[i].next)
         {
             Edge &e=edge[i];
             if (dis[e.to]==dis[x]+ && (f=dfs(e.to,min(a,e.cap-e.flow)))>)
             {
                 e.flow+=f;
                 edge[i^].flow-=f;
                 a-=f;
                 flow+=f;
                 if (!a) break;
             }
         }
         return flow;
     }
     int Dinic(int s,int t)
     {
         this->s=s,this->t=t;
         int ans=;
         while (bfs())
         {
             for (int i=;i<=n;i++) cur[i]=first[i];
             ans+=dfs(s,inf);
         }
         return ans;
     }
 }g;
 int x,y;
 int main()
 {
     scanf("%d%d",&n,&m);
     g.clear(n*+);
     int s=g.n-,t=g.n;
     for (int i=;i<=n;i++) g.insert(s,i,);
     for (int i=n+;i<=n*;i++) g.insert(i,t,);
     for (int i=;i<=m;i++)
     {
         scanf("%d%d",&x,&y);
         g.insert(x,y+n,);
     }
     printf("%d\n",g.Dinic(s,t));
     return ;
 }