long long 阶乘(int n)
{
long long a = 1;
while (n)a *= n--;
return a;
}
void 编写函数根据公式求(int m,int n)
{//缘由https://ask.youkuaiyun.com/questions/7439058?spm=1005.2025.3001.5141
double a = 1.0;
m = 阶乘(m - n);
n = 阶乘(n);
std::cout << (a = a * m / n) << std::endl;
}