input: Write a function to reverse the order of words in a given sentence while keeping the order of characters in each word unchanged. If the input string is empty, return an empty string.
  assert reverse_words("This is a sentence.") == "sentence. a is This"
  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):
      words = s.split()
      reversed_words = words[::-1]
      return ' '.join(reversed_words)