input: Write a function to reverse the order of words in a given sentence while keeping the characters in each word in the same order. The function should take a string as input and return the modified string.
  
  def reverse_words(string):
      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 programming is fun") == "fun is programming Python"
  assert reverse_words("SingleWord") == "SingleWord"
  assert reverse_words("") == ""
output: def reverse_words(string):
      words = string.split(" ")
      reversed_words = []
      for word in words:
          reversed_words.insert(0, word)
      return ' '.join(reversed_words)