input: Write a function to check if a given string is a palindrome.
  assert is_palindrome('') == True
  assert is_palindrome('aba') == True
  assert is_palindrome('aaaaa') == True
  assert is_palindrome('zbcd') == False
  assert is_palindrome('xywyx') == True
  assert is_palindrome('xywyz') == False
  assert is_palindrome('xywzx') == False
  
output: def is_palindrome(s):
      return s == s[::-1]