input: Write a function to calculate the factorial of a given number using recursion. The function should return 1 if the input number is 0.
  
  def factorial(n):
   if n == 0:
    return 1
   else:
    return n * factorial(n-1)
  
  assert factorial(0)==1
  assert factorial(5)==120
  assert factorial(10)==3628800
output: def factorial(n):
   if n == 0:
    return 1
   else:
    return n * factorial(n-1)