并查集+组合数学

A. Bear and Friendship Condition 并查集：每个集合的个数，如果集合若是有向强连通分量，则集合内边数必须满足n*(n-1)/2

#include 
#define int long long
using namespace std;
const int N=2e5+5;
const int mod=1e9+7;
int f[N],n,m,ans,sz[N];
int r_find(int r)
{
    if(r==f[r])
        return f[r];
    f[r]=r_find(f[r]);
    sz[r]=sz[f[r]];
    return f[r];
}
signed main()
{
    cin>>n>>m;
    for(int i=1;ix>>y;
        if(x>y) swap(x,y);
        int fx=r_find(x),fy=r_find(y);
        if(fx==fy)
            continue;
        f[fx]=fy;
        sz[fx]+=sz[fy];
        sz[fy]=sz[fx];
    }
    int flag=0;
    for(int i=1;i