input: Write a function to determine if a string `s` is a palindrome. A palindrome is a string that reads the same forwards and backwards. If `s` is a palindrome, return True; otherwise, return False. If the string `s` is empty, return True.
  
  assert is_palindrome("racecar")==True
  assert is_palindrome("hello")==False
  assert is_palindrome("madam")==True
  assert is_palindrome("12321")==True
  assert is_palindrome("")==True
output: def is_palindrome(s):
      return s == s[::-1]