input: Write a function to calculate the factorial of a number using recursion. The function should take an integer as input and return the factorial of that number. If the input number is 1, the function should return 1.
  
  assert factorial(5)==120
  assert factorial(1)==1
  assert factorial(3)==6
  assert factorial(0)==1
  assert factorial(4)==24
output: def factorial(n):
      if n == 0 or n == 1:
          return 1
      else:
          return n * factorial(n - 1)