nyoj 光棍节的快乐

http://acm.nyist.net/JudgeOnline/problem.php?pid=451

#include<stdio.h>
long long  a[22],b[22];
void cuo_pai()
{
    a[1]=0;
    a[2]=1;
    for(int i=3;i<=22;i++)
    a[i]=(i-1)*(a[i-1]+a[i-2]);
}
int main()
{
    long long aa,b;
    cuo_pai();
    while(~scanf("%lld%lld",&aa,&b))
    {
    //    printf("%d\n",a[b]);
        long long temp=b;
        long long ccc=b;
        long long sum=1;
        while(b)
        {
            sum*=aa;
            aa--;
            b--;
        }
        while(temp)
        {
            sum/=temp;
            temp--;
        }
        printf("%lld\n",sum*a[ccc]);
    }
}