input: Write a function to calculate the factorial of a given number using an iterative approach. The function should take an integer as input, and return the factorial of that number. If the input number is less than 0, the function should return 1. 
  
  assert factorial(7)==5040
  assert factorial(0)==1
  assert factorial(-1)==1
  assert factorial(5)==120
output: def factorial(n):
      if n < 0:
          return 1
      result = 1
      for i in range(2, n + 1):
          result *= i
      return result