input: Write a function to reverse the order of words in a given sentence while keeping the order of characters in each word the same. 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('Hello World')=='World Hello'
  assert reverse_words('Python is fun')=='fun is Python'
  assert reverse_words('')==''
  assert reverse_words('SingleWord')=='SingleWord'
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)