input: Write a function to reverse the order of words in a given sentence while keeping the order of characters in each word unchanged.
  assertEqual reverse_words('This is a sentence.') == 'sentence. a is This'
  assertEqual reverse_words('Python programming is fun') == 'fun is programming Python'
  assertEqual reverse_words('') == ''
  assertEqual reverse_words('SingleWord') == 'SingleWord'
  assertEqual reverse_words('Word1 Word2 Word3') == 'Word3 Word2 Word1'
output: def reverse_words(sentence):
      words = sentence.split()
      reversed_words = words[::-1]
      return ' '.join(reversed_words)