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 or less, the function should return 1. The function should be named factorial and take one parameter n.
  assert factorial(5)==120
  assert factorial(1)==1
  assert factorial(0)==1
  assert factorial(-3)==1
  assert factorial(6)==720
output: def factorial(n):
      if n <= 1:
          return 1
      else:
          return n * factorial(n - 1)