input: Write a function that takes an integer as input and returns the factorial of that integer. If the input is less than or equal to 1, return 1. 
  assert factorial(5)==120
  assert factorial(3)==6
  assert factorial(1)==1
  assert factorial(0)==1
  assert factorial(-5)==1
output: def factorial(n):
      if n <= 1:
          return 1
      result = 1
      for i in range(2, n + 1):
          result *= i
      return result