input: Write a function to reverse the order of words in a sentence while keeping the order of characters within each word unchanged. If the input string is empty, return the empty string.
  assert reverse_words("This is a sentence.") == "sentence. a is This"
  assert reverse_words("Hello World") == "World Hello"
  assert reverse_words("") == ""
  assert reverse_words("One") == "One"
  assert reverse_words("Python is fun") == "fun is Python"
output: def reverse_words(s):
      words = s.split()
      return ' '.join(words[::-1])