主题链接：

题意：白书的P103.

加个虚根就能够了。。。然后就是一个多重集排列。

import java.io.PrintWriter;import java.util.ArrayList;import java.util.Scanner;public class Main {	static int N = 40100;	ArrayList
    
     [] G = new ArrayList[N];	static long mod = 1000000007;	long[] way = new long[N];	long[] fac = new long[N];	int n, m;	int[] father = new int[N], siz = new int[N];	long Pow(long x, long y){		long ans = 1;		while(y > 0){			if(y%2 == 1L)				ans = (ans*x)%mod;			x = (x*x)%mod;			y >>= 1;		}		return ans;	}	void dfs(int u, int fa){		siz[u] = 1; way[u] = 1L;		for(int i = 0; i < G[u].size(); i++){			int v = G[u].get(i); if(v == fa)continue;			dfs(v, u);			siz[u] += siz[v];			way[u] = (way[u]*way[v])%mod;		}		way[u] = (way[u]* fac[siz[u]-1]) %mod;		for(int i = 0; i < G[u].size(); i++){			int v = G[u].get(i); if(v == fa)continue;			way[u] = (way[u] * Pow(fac[siz[v]], mod-2))%mod;		}	}	void init(){		n = cin.nextInt();	m = cin.nextInt();		for(int i = 0; i <= n; i++){			father[i] = 0;			G[i].clear();		}		while(m-- > 0){			int u = cin.nextInt(), v = cin.nextInt();			G[v].add(u);			father[u] = v;		}		for(int i = 1; i <= n; i++)			if(father[i] == 0){				G[0].add(i);			}	}	void work() {				for(int i = 0; i < N; i++)G[i] = new ArrayList();		fac[0] = 1L;		for(int i = 1; i < N; i++)	fac[i] = fac[i-1]*i%mod;		int T = cin.nextInt();		while(T-->0){			init();			dfs(0,0);			out.println(way[0]);		}	}	Main() {		cin = new Scanner(System.in);		out = new PrintWriter(System.out);	}	public static void main(String[] args) {		Main e = new Main();		e.work();		out.close();	}	public Scanner cin;	public static PrintWriter out;}