input: Write a function to check if a given string is a palindrome. A palindrome is a string that reads the same forward and backward. If the string is a palindrome, return True; otherwise, return False. If the string is empty, 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("level")==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()
    return s == s[::-1]