input: Write a function to calculate the factorial of a number using recursion in Python 3. If the given number is 0, then return 1.
  assert factorial(0)==1
  assert factorial(1)==1
  assert factorial(5)==120
  assert factorial(10)==3628800
  assert factorial(7)==5040
output: def factorial(n):
      if n == 0:
          return 1
      else:
          return n * factorial(n - 1)