洛谷P2881 [USACO07MAR]排名的牛Ranking the Cows(bitset Floyd)

#include<bits/stdc++.h>
using namespace std;
const int MAXN = 1001;
inline int read() {
    char c = getchar(); int x = 0, f = 1;
    while(c < '0' || c > '9') {if(c == '-') f = -1; c = getchar();}
    while(c >= '0' && c <= '9') x = x * 10 + c - '0', c = getchar();
    return x * f;
}
int N, M;
bitset<MAXN> f[MAXN];
int main() {
    N = read(); M = read();
    for(int i = 1; i <= M; i++) {
        int x = read(), y = read();
        f[x][y] = 1;
    }
    for(int k = 1; k <= N; k++)
        for(int i = 1; i <= N; i++)
            if(f[i][k]) f[i] = f[i] | f[k];
    int ans = N * (N - 1) / 2;
    for(int i = 1; i <= N; i++) ans -= f[i].count();
    cout << ans;
    return 0;
}