c 语言，第4小题，思路不是很懂，解答的大神能发代码上来吗？

Question

Answer 1

阶乘函数Factorial

 double Factorial(double n)
 {
      if(n==0)return 1;
        else return Factorial(n-1)
 }

组合函数Combination

 double Combination(double n,double k)
 {
     return Factorial(n)/(Factorial(k)*Factorial(n-k));
 }