input: Write a function that calculates the factorial of a given number using recursion. The function should return the factorial of the provided number. If the given number 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(3)==6
  assert factorial(4)==24
  assert factorial(2)==2
output: def factorial(n):
   if n == 0:
    return 1
   else:
    return n * factorial(n-1)