input: Write a function to reverse the order of words in a given sentence while keeping the order of characters within each word unchanged. Return the modified sentence.
  def reverse_words(sentence):
    words = sentence.split(" ")
    reversed_words = []
    for word in words:
      reversed_words.insert(0, word)
    return ' '.join(reversed_words)
  
  assert reverse_words('This is a test') == 'test a is This'
  assert reverse_words('Python programming is fun') == 'fun is programming Python'
  assert reverse_words('Hello world') == 'world Hello'
  assert reverse_words('SingleWord') == 'SingleWord'
  assert reverse_words('') == ''
output: def reverse_words(sentence):
    words = sentence.split(" ")
    reversed_words = []
    for word in words:
      reversed_words.insert(0, word)
    return ' '.join(reversed_words)