input: Write a function to check if a given string is a palindrome. The function should return True if the string is a palindrome, and False otherwise. If the string is empty, it should return True.
  assert is_palindrome("racecar")==True
  assert is_palindrome("hello")==False
  assert is_palindrome("")==True
  assert is_palindrome("a")==True
output: def is_palindrome(s):
      return s == s[::-1]