input: Write a function to understand the concept of recursion in Python by defining a recursive function to calculate the factorial of a number. If the number is 1, return 1. Otherwise, return the number multiplied by the factorial of the number minus one.
  assert factorial(5)==120
  assert factorial(1)==1
  assert factorial(0)==1
output: def factorial(n):
      if n == 0 or n == 1:
          return 1
      else:
          return n * factorial(n - 1)