input: Write a function to reverse the order of words in a given string. The function should take a string as input and return a string with the words in reverse order. If the input string is empty, the function should return an empty string.
  assert reverse_words("Hello world")=="world Hello"
  assert reverse_words("Python is great")=="great is Python"
  assert reverse_words("One two three")=="three two One"
  assert reverse_words("")==""
output: def reverse_words(s):
      if not s:
          return ""
      words = s.split()
      return " ".join(words[::-1])