input: Write a function to determine if a given string is a palindrome. If the string is a palindrome, return True; otherwise, return False. If the given string is empty, return True.
  assert is_palindrome("racecar")==True
  assert is_palindrome("hello")==False
  assert is_palindrome("madam")==True
  assert is_palindrome("level")==True
  assert is_palindrome("")==True
output: def is_palindrome(s):
      return s == s[::-1]