input: Write a function to calculate the factorial of a given number. If the number is 0, return 1.
  def factorial(n):
   if n == 0:
    return 1
   else:
    return n * factorial(n-1)
  assert factorial(5)==120
  assert factorial(3)==6
  assert factorial(0)==1
  assert factorial(1)==1
output: def factorial(n):
   if n == 0:
    return 1
   else:
    return n * factorial(n-1)