SPOJ 78 Marbles 组合数学

相当于从n-1个位置里面找k-1个位置放隔板

 

#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <algorithm>

#define LL long long int

using namespace std;

int main()
{
    double n, k;
    int T;
    scanf( "%d", &T );
    while ( T-- )
    {
        scanf( "%lf%lf", &n, &k );
        if ( n - k < k - 1 ) k = n - k + 1.0;
        double ans = 1.0;
        for ( double i = 1; i < k; ++i )
        {
            ans *= ( n - i ) / i;
        }
        printf( "%.0f\n", ans );
    }
    return 0;
}