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