input: Write a function to check if a given string `s` is a palindrome. A palindrome is a string that reads the same forward and backward. Return `True` if the string is a palindrome, otherwise return `False`. If the string is empty, return `True`.
  
  def is_palindrome(s):
      '''Write your code here.'''
  
  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]