input: Write a function to calculate the factorial of a given number using recursive approach. The factorial of a number is the product of all positive integers less than or equal to that number. If the given number is 0, the factorial is 1.
  def factorial(n):
   if n == 0:
    return 1
   else:
    return n * factorial(n-1)
  assert factorial(5)==120
  assert factorial(0)==1
  assert factorial(3)==6
  assert factorial(1)==1
  assert factorial(4)==24
output: def factorial(n):
   if n == 0:
    return 1
   else:
    return n * factorial(n-1)