poj 3041 Asteroids

#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<vector>
#include<queue>
#include<algorithm>
#define re return
#define inc(i,l,r) for(int i=l;i<=r;++i)
const int maxn=1505,maxm=20005;
using namespace std;
template<typename T>inline void rd(T&x)
{
	char c;bool f=0;
	while((c=getchar())<‘0‘||c>‘9‘)if(c==‘-‘)f=1;
	x=c^48;
	while((c=getchar())>=‘0‘&&c<=‘9‘)x=x*10+(c^48);
	if(f)x=-x;
} 

int n,m,s,t,deep[maxn],k=1,hd[maxn],cur[maxn];
struct node{
	int to,nt,w;
}e[maxm];

inline void add(int x,int y,int z)
{
	e[++k].to=y;e[k].nt=hd[x];hd[x]=k;e[k].w=z;
	e[++k].to=x;e[k].nt=hd[y];hd[y]=k;e[k].w=0;
}

inline bool  bfs()
{
	queue<int>q;
	inc(i,1,t)deep[i]=0;
	deep[s]=1;
	q.push(s);
	while(!q.empty())
	{
		int u=q.front();
		q.pop();
		for(int i=hd[u];i;i=e[i].nt)
		{
			int v=e[i].to;
			if(!deep[v]&&e[i].w)
			{
				deep[v]=deep[u]+1;
				if(v==t)re 1;
				q.push(v);
			} 
		}
	}
	re 0;
}

inline int dfs(int u,int flow)
{
	if(u==t) re flow;
	int delta=flow;
	for(int &i=cur[u];i;i=e[i].nt)
	{
		int v=e[i].to;
		if(deep[v]==deep[u]+1&&e[i].w)
		{
			int d=dfs(v,min(delta,e[i].w));
			e[i].w-=d;e[i^1].w+=d;
			delta-=d;
			if(!delta)re flow;
		}
	}
	re flow-delta;
}

int main()
{
	rd(n),rd(m);
	s=(n<<1)+1;
	t=s+1;
	int x,y;
	inc(i,1,m)
	{
		rd(x),rd(y);
		add(x,y+n,1);
	}
	inc(i,1,n){
		add(s,i,1);
		add(i+n,t,1);
	}
	int ans=0;
	while(bfs())
	{
		inc(i,1,t)cur[i]=hd[i];
		ans+=dfs(s,n);
	} 
	printf("%d",ans);
	re 0;
}