input: Write a function to calculate the factorial of a number using a recursive approach. The function should return 1 if the input number is 0.
  assert factorial(0)==1
  assert factorial(1)==1
  assert factorial(5)==120
  assert factorial(10)==3628800
  assert factorial(3)==6
output: def factorial(n):
      if n == 0:
          return 1
      else:
          return n * factorial(n - 1)