input: Write a python function to check if a string is a palindrome.
  assert is_palindrome("racecar") == True
  assert is_palindrome("hello") == False
  assert is_palindrome("madam") == True
  assert is_palindrome("step on no pets") == True
output: def is_palindrome(s: str) -> bool:
      cleaned_str = ''.join(c for c in s if c.isalnum()).lower()
      return cleaned_str == cleaned_str[::-1]