input: Write a function to reverse the order of words in a given sentence while keeping the order of characters in each word unchanged. The function should return the modified sentence.
  
  def reverse_words(sentence):
      words = sentence.split()
      reversed_sentence = ' '.join(words[::-1])
      return reversed_sentence
  
  assert reverse_words("This is a sentence.") == "sentence. a is This"
  assert reverse_words("Hello world") == "world Hello"
  assert reverse_words("The quick brown fox") == "fox brown quick The"
  assert reverse_words("") == ""
  assert reverse_words("SingleWord") == "SingleWord"
output: def reverse_words(sentence):
      words = sentence.split()
      reversed_sentence = ' '.join(words[::-1])
      return reversed_sentence