laoshi讲完P2419后我花了五分钟一口气敲出来了这段AC代码，有同学吐槽说码风潦草，怎么可能？？

代码见下：

#include<iostream>
#define re register
using namespace std;

const int N = 5e3 + 5;

int n,m,ans;
bool dp[N][N];

int main(){
	ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);
	cin>>n>>m;
	for(re int i = 1;i <= m;i ++){
		int a,b;
		cin>>a>>b;
		dp[a][b] = true;
	}
	for(re int k = 1;k <= n;k ++){
		for(re int i = 1;i <= n;i ++){
			for(re int j = 1;j <= n;j ++){
				dp[i][j] = dp[i][j] || (dp[i][k] && dp[k][j]);
			}
		}
	}
	for(re int i = 1;i <= n;i ++){
		int tmp = 1;
		for(re int j = 1;j <= n;j ++){
			if(i == j){
				continue;
			}
			tmp = tmp && (dp[i][j] || dp[j][i]);
		}
		ans += tmp;
	}
	cout<<ans<<"\n";
	return 0;
}