input: Write a function to calculate the factorial of a given integer. The function should return the factorial of the integer. If the input number is 1 or less, the function should return 1. The function should be called factorial and take an integer as its parameter.
  assert factorial(7) == 5040
  assert factorial(5) == 120
  assert factorial(0) == 1
  assert factorial(1) == 1
  assert factorial(3) == 6
output: def factorial(n):
      if n <= 1:
          return 1
      result = 1
      for i in range(2, n + 1):
          result *= i
      return result