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