input: Write a function to determine whether a given string `s` is a palindrome. A palindrome is a word, phrase, number, or other sequence of characters that reads the same backward as forward. The function should return `True` if `s` is a palindrome and `False` otherwise.
  assert is_palindrome("racecar")==True
  assert is_palindrome("hello")==False
  assert is_palindrome("madam")==True
  assert is_palindrome("12321")==True
  assert is_palindrome("")==True
output: def is_palindrome(s):
      return s == s[::-1]