P7368 [USACO05NOV]Asteroids G(二分图最大独立集)

Posted 2021-08-03 Harris-H

P7368 [USACO05NOV]Asteroids G(二分图最大独立集)

比较经典的二分图最大独立集建模问题。

code

// Problem: P7368 [USACO05NOV]Asteroids G
// Contest: Luogu
// URL: https://www.luogu.com.cn/problem/P7368
// Memory Limit: 256 MB
// Time Limit: 1000 ms
// Date: 2021-07-06 09:21:47
// --------by Herio--------

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef unsigned long long ull; 
const int N=505,M=3e5+5,inf=0x3f3f3f3f,mod=1e9+7;
#define mst(a,b) memset(a,b,sizeof a)
#define PII pair<int,int>
#define fi first
#define se second
#define pb emplace_back
#define SZ(a) (int)a.size()
#define ios ios::sync_with_stdio(false),cin.tie(0) 
void Print(int *a,int n){
	for(int i=1;i<n;i++)
		printf("%d ",a[i]);
	printf("%d\\n",a[n]); 
}
int n,k;
bitset<N>vis;
struct edge{
	int to,nt;
}e[M];
int h[N],cnt,mh[N];
void add(int u,int v){
	e[++cnt]={v,h[u]},h[u]=cnt;
}
bool find(int u){
	for(int i=h[u];i;i=e[i].nt){
		int v=e[i].to;
		if(vis[v]) continue;
		vis[v]=1;
		if(!mh[v]||find(mh[v])){
			mh[v]=u;
			return true;
		}
	}
	return false;
}
int main(){
	scanf("%d%d",&n,&k);
	//set<int>s;
	for(int i=1;i<=k;i++){
		int x,y;
		scanf("%d%d",&x,&y);
	//	s.insert(x);
		add(x,y);
	}
	int ans=0;
	for(int i=1;i<=n;i++) {
		vis.reset();if(find(i)) ans++;
	}
	/*for(int x:s){
		vis.reset();
		if(find(x)) ans++;
	}*/
	printf("%d\\n",ans);
	return 0;
}