UVa 11174 - Stand in a Line

#include <iostream>
#include <cstdio>
#include <string>
#include <cstring>
#include <cmath>
#include <algorithm>
#include <queue>

#define ll long long
using namespace std;

const ll MOD=1000000007;
const int N=100005;
ll power(ll x,ll y)
{
ll tmp=1;
while(y)
{
if(y&1)
tmp=tmp*x%MOD;
x=x*x%MOD;
y=y>>1;
}
return tmp;
}
int num
,f
,sum
;
queue<int>qt;
int main()
{
//freopen("data.in","r",stdin);
int T;
cin>>T;
while(T--)
{
int n,m;
cin>>n>>m;
memset(num,0,sizeof(num));
memset(sum,0,sizeof(sum));
memset(f,0,sizeof(f));
while(!qt.empty()) qt.pop();
while(m--)
{
int a,b;
cin>>a>>b;
++num[b];
f[a]=b;
}
for(int i=1;i<=n;++i)
if(num[i]==0)
qt.push(i);

ll x=1,y=1;
while(!qt.empty())
{
int i=qt.front();qt.pop();
++sum[i];
y=y*sum[i]%MOD;
sum[f[i]]+=sum[i];
if((--num[f[i]])==0)
qt.push(f[i]);
}
for(int i=1;i<=n;++i)
x=x*i%MOD;
cout<<(x*power(y,MOD-2))%MOD<<endl;
}
return 0;
}