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):
      cleaned = ''.join(c for c in s if c.isalnum()).lower()
      return cleaned == cleaned[::-1]