题解 | #牛牛的排列数#

#include <stdio.h>

unsigned long int A(unsigned long int n, unsigned long int m);
unsigned long int Fac(unsigned long int n);

int main() {
    unsigned long int n = 0;
    unsigned long int m = 0;

    scanf("%lu %lu", &n, &m);
    if (n < m)
    {
        printf("0");
    }
    else 
    {
        printf("%lu", A(n, m));
    }

    return 0;
}

unsigned long int A(unsigned long int n, unsigned long int m)
{
	return Fac(n) / Fac(n - m);
}

unsigned long int Fac(unsigned long int n)
{
	if (n <= 1)
	{
		return 1;
	}
	else
	{
		return n * Fac(n - 1);
	}
}