input: Write a function to understand and demonstrate the concept of recursion in Python. Implement a recursive function to calculate the factorial of a given number n. If n is 1, return 1; otherwise, return n multiplied by the factorial of n - 1. Test the function by calculating the factorial of 5 and print the result.
  assert factorial(5)==120
  assert factorial(1)==1
  assert factorial(0)==None
  assert factorial(3)==6
  assert factorial(10)==3628800
output: def factorial(n):
      if n == 1:
          return 1
      elif n == 0:
          return None
      else:
          return n * factorial(n - 1)