input: Write a python function to check if a given string is a palindrome.
  assert is_palindrome("racecar") == True
  assert is_palindrome("hello") == False
  assert is_palindrome("A man a plan a canal Panama") == True
output: def is_palindrome(s):
      cleaned = ''.join(c.lower() for c in s if c.isalnum())
      return cleaned == cleaned[::-1]