POJ 3041.Asteroids 最小顶点覆盖

#include<iostream>
#include<cstdio>
#include<cmath>
#include<cstring>
#include<algorithm>
#include<set>
#include<map>
#include<queue>
#include<stack>
#include<vector>
using namespace std;
#define PI acos(-1.0)
typedef long long ll;
typedef pair<int,int> P;
const int maxn=1e5+100,maxm=1e5+100,inf=0x3f3f3f3f,mod=1e9+7;
const ll INF= 1e13+7;
priority_queue<P,vector<P>,greater<P> >q;
vector<int>G[maxn];
int vis[maxn],cy[maxn];
int dfs(int u)
{
    for(int i=0; i<G[u].size(); i++)
    {
        int v=G[u][i];
        if(vis[v]) continue;
        vis[v]=true;
        if(cy[v]==-1||dfs(cy[v]))
        {
            cy[v]=u;
            return true;
        }
    }
    return false;
}
int solve(int n)
{
    int ans=0;
    memset(cy,-1,sizeof(cy));
    for(int i=1; i<=n; i++)
    {
        memset(vis,0,sizeof(vis));
        ans+=dfs(i);
    }
    return ans;
}
int main()
{
    int n,k;
    scanf("%d%d",&n,&k);
    for(int i=1; i<=k; i++)
    {
        int x,y;
        scanf("%d%d",&x,&y);
        G[x].push_back(y);
    }
    cout<<solve(n)<<endl;
    return 0;
}