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