hdu 2049 考新郎

#include<iostream>
#include<cmath>
#include<iomanip>
using namespace std;

long long fact(int n)
{
long long sum =1;
while(n>0)
sum*=n--;
return sum;
}

int main()
{
int n;
cin>>n;
while(n--)
{
int m,n;
cin>>m>>n;
long long a[21];
a[1] = 0;
a[2] = 1;
for(int i = 3;i<=n;i++)
{
a[i] = (i-1)*(a[i-1]+a[i-2]);
}
cout<<a
*fact(m)/(fact(n)*fact(m-n))<<endl;
}
}