input: Write a function to determine if a given string is a palindrome. The function should return True if the string is a palindrome, and False otherwise. If the string is empty, it should return True.
  
  def is_palindrome(s):
    s = s.lower()
    left = 0
    right = len(s) - 1
    
    while left < right:
      if s[left] != s[right]:
        return False
      left += 1
      right -= 1
    return True
  
  assert is_palindrome("racecar")==True
  assert is_palindrome("hello")==False
  assert is_palindrome("A man, a plan, a canal, Panama")==True
  assert is_palindrome("")==True
output: def is_palindrome(s):
    s = ''.join(filter(str.isalnum, s)).lower()
    left = 0
    right = len(s) - 1
    
    while left < right:
      if s[left] != s[right]:
        return False
      left += 1
      right -= 1
    return True