bzoj1703[Usaco2007 Mar]Ranking the Cows 奶牛排名

#include<cstdio>
#include<cstring>
#include<algorithm>
#include<bitset>
#define fo(i,a,b) for(int i=a;i<=b;i++)
#define fd(i,a,b) for(int i=a;i>=b;i--)
using namespace std;
const int N=1e3+5;
int n,m;
bitset<N> a
;
int main()
{
scanf("%d%d",&n,&m);
fo(i,1,m)
{
int x,y;
scanf("%d%d",&x,&y);
a[x][y]=1;
}
fo(i,1,n)
4000

fo(j,1,n)
if (a[j][i])a[j]|=a[i];
int ans=0;
fo(i,1,n)
fo(j,i+1,n)
if (!a[i][j]&&!a[j][i])ans++;
printf("%d\n",ans);
}