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.
  def reverse_words(string):
    if string == "":
      return ""
    words = string.split(" ")
    reversed_words = []
    for word in words:
      reversed_words.insert(0, word)
    return ' '.join(reversed_words)
  
  assert reverse_words('This is a sentence.') == 'sentence. a is This'
  assert reverse_words('Python programming is fun') == 'fun is programming Python'
  assert reverse_words('') == ''
  assert reverse_words('Hello') == 'Hello'
  assert reverse_words('Single word') == 'word Single'
output: def reverse_words(string):
    if string == "":
      return ""
    words = string.split(" ")
    reversed_words = []
    for word in words:
      reversed_words.insert(0, word)
    return ' '.join(reversed_words)