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 given sentence is empty, return an empty string.
  
  def reverse_words(sentence):
    if sentence == "":
      return ""
    words = sentence.split(" ")
    reversed_words = words[::-1]
    return ' '.join(reversed_words)
  
  assert reverse_words('This is a sentence.')=='sentence. a is This'
  assert reverse_words('Another example here.')=='here. example Another'
  assert reverse_words('Single')=='Single'
  assert reverse_words('')==''
  assert reverse_words('Two words')=='words Two'
output: def reverse_words(sentence):
    if sentence == "":
      return ""
    words = sentence.split(" ")
    reversed_words = words[::-1]
    return ' '.join(reversed_words)