input: Write a function that checks if a given string is a palindrome (reads the same forwards and backwards) after converting all uppercase letters into lowercase and removing all non-alphanumeric characters. If the input is a palindrome, return True; otherwise, return False. If the input is an empty string, return True.
  assert is_palindrome("A man, a plan, a canal: Panama")==True
  assert is_palindrome("race a car")==False
  assert is_palindrome("No 'x' in Nixon")==True
  assert is_palindrome("12321")==True
  assert is_palindrome("")==True
output: def is_palindrome(s: str) -> bool:
      cleaned_str = ''.join(char.lower() for char in s if char.isalnum())
      return cleaned_str == cleaned_str[::-1]