input: Write a function to calculate the factorial of a given number using recursion.
  
  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)