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;

}