input: Write a function to understand recursion in Python by implementing a recursive function to calculate the factorial of a given number. Return the factorial of the number.
  assert factorial(5)==120
  assert factorial(1)==1
  assert factorial(0)==1
  assert factorial(3)==6
  assert factorial(10)==3628800
output: def factorial(n):
      if n == 0 or n == 1:
          return 1
      else:
          return n * factorial(n - 1)