input: Write a function that calculates the factorial of a given number 'n'. The factorial of a number is the product of an integer and all the integers below it. If 'n' is 0, the function should return 1.
  
  def factorial(n):
   if n == 0:
    return 1
   else:
    return n * factorial(n-1)
  
  assert factorial(0)==1
  assert factorial(1)==1
  assert factorial(5)==120
  assert factorial(10)==3628800
output: def factorial(n):
   if n == 0:
    return 1
   else:
    return n * factorial(n-1)