input: Write a function to understand the concept of recursion in Python by calculating the factorial of a number using a recursive function.
  def factorial(n):
     if n == 1:
        return 1
     else:
        return n * factorial(n - 1)
  assert factorial(1)==1
  assert factorial(5)==120
  assert factorial(0)==1
output: def factorial(n):
     if n == 0 or n == 1:
        return 1
     else:
        return n * factorial(n - 1)