input: Write a function to determine if a given string is a palindrome using recursion. The function should return True if the string is a palindrome and False otherwise. If the input string is empty, return True.
  assert is_palindrome("racecar")==True
  assert is_palindrome("hello")==False
  assert is_palindrome("madam")==True
  assert is_palindrome("a")==True
  assert is_palindrome("")==True
output: def is_palindrome(s):
      if len(s) <= 1:
          return True
      if s[0] != s[-1]:
          return False
      return is_palindrome(s[1:-1])