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 new string with the words in the reversed order. If the input string is empty, the function should return an empty string.
  assert reverse_words("Hello world this is a test") == "test a is this world Hello"
  assert reverse_words("Python is fun") == "fun is Python"
  assert reverse_words("SingleWord") == "SingleWord"
  assert reverse_words("") == ""
output: def reverse_words(s):
      words = s.split()
      return ' '.join(words[::-1])