light oj 1005 - Rooks(组合数学)

#include <stdio.h>
#include <string.h>

typedef __int64 LL;

LL A(int n, int m)
{
    LL ans = 1;
    for (int i = n-m+1; i <= n; i++)
        ans *= i;
    return ans;
}

LL C(int n, int m)
{
    LL ans = 1;
    for (int i = 1; i <= m; i++)
        ans = ans*(n-i+1)/i;
    return ans;
}

int main()
{
    int t, m, n;
    scanf("%d", &t);
    for (int i = 1; i <= t; i++)
    {
        LL ans;
        scanf("%d %d", &n, &m);
        if (m <= n)
            ans = C(n, m)*A(n, m);
        else
            ans = 0;
        printf("Case %d: %I64d\n",i, ans);
    }
    return 0;
}