input: Write a function to determine if a given string is a palindrome, which means it reads the same forwards and backwards. 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("level")==True
  assert is_palindrome("")==True
  assert is_palindrome("a")==True
output: def is_palindrome(s):
      return s == s[::-1]