uva 11174

刘书上例题

#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <vector>
#include <sstream>
#include <string>
#include <cstring>
#include <algorithm>
#include <iostream>
#define maxn 40010
#define INF 0x7fffffff
#define inf 10000000
#define MOD 1000000007
#define ull unsigned long long
#define ll long long
using namespace std;

ll fac[maxn];
int c[maxn];
bool vis[maxn];
vector<int> g[maxn];

void init()
{
    fac[1] = 1;
    for(int i = 2; i < maxn; ++ i) fac[i] = fac[i-1]*i%MOD;
}

ll pow_mod(ll a, ll p, ll n)
{
    if(p == 0) return 1;
    ll ans = pow_mod(a, p/2, n);
    ans = ans*ans%n;
    if(p%2) ans = ans*a%n;
    return ans;
}

void dfs(int u)
{
    c[u] = 1;
    vis[u] = true;
    for(int i = 0; i < (int)g[u].size(); ++ i)
    {
        if(!vis[g[u][i]]) dfs(g[u][i]);
        c[u] += c[g[u][i]];
    }
}

int main()
{
    int t, m, n;
    scanf("%d", &t);
    init();
    while(t --)
    {
        memset(vis, 0, sizeof(vis));
        scanf("%d%d", &n, &m);
        for(int i = 1; i <= n; ++ i) g[i].clear();
        for(int i = 0; i < m; ++ i)
        {
            int u, v;
            scanf("%d%d", &u, &v);
            g[v].push_back(u);
        }
        for(int i = 1; i <= n; ++ i)
            if(!vis[i]) dfs(i);
        long long ans = fac[n];
        for(int i = 1; i <= n; ++ i)
        {
            long long k = pow_mod(c[i], MOD - 2, MOD);
            ans = ans * k % MOD;
        }
        printf("%lld\n", ans);
    }
    return 0;
}