学步园

#include<iostream>
#include<algorithm>
#include<cstring>
#include<cstdio>
#define min(a,b) (a)>(b)?(b):(a)
#define max(a,b) (a)>(b)?(a):(b)
#define inf 99999999
using namespace std;
int cnt;
int source,N;
int sink,ans;
int c[4400];
int H[20410];
int d[4410];
struct egde
{
    int u,v,c,next;
};
egde a[20000];
void init()
{
    int i;
    for(i=0;i<24000;i++)
        H[i]=-1;
    cnt=0;
}
void add(int u,int v,int c)
{
    a[cnt].v=v;
    a[cnt].c=c;
    a[cnt].next=H[u];
    H[u]=cnt++;
}
void build(int u,int v,int c)
{
    add(u,v,c);
    add(v,u,0);
}
int dfs(int u,int flow)
{  
    if(u==sink)
        return flow;
    int tt,res=0,delta;
    for(tt=H[u];~tt;tt=a[tt].next)
    {
        int &v=a[tt].v,&c=a[tt].c;
        if(c&&d[u]==d[v]+1){
        delta=dfs(v,min(flow,c));
        c-=delta;
        a[tt^1].c+=delta;
        res+=delta;
        flow-=delta;
        if (!flow)
             return res;
        }
    }
    if(!res)
    {
        if(!--c[d[u]]) 
            d[source]=N+1;
        ++c[++d[u]];
    }
    return res;
}
void sap()
{
    memset(c,0,sizeof(c)); 
    memset(d,0,sizeof(d));
    c[source]=N;
    while(d[source]<N)
        ans+=dfs(source,inf);
}
int main()
{
	int n,k,i,x,y;
	while(~scanf("%d%d",&n,&k))
	{
		init();
		source=0;
		sink=n*2+1;
		N=sink+1;
		ans=0;
		for(i=1;i<=k;i++)
		{
			scanf("%d%d",&x,&y);
			build(x,y+n,inf);
		}
		for(i=1;i<=n;i++)
		{
			build(source,i,1);
			build(i+n,sink,1);	
		}
		sap();
		printf("%d\n",ans);
	}
	return 0;
}