input: Write a function to calculate the factorial of a given integer. The function should take an integer as input and return the factorial of that integer. If the input integer is less than or equal to 1, the function should return 1.
  assert factorial(7)==5040
  assert factorial(0)==1
  assert factorial(1)==1
  assert factorial(-5)==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