input: Write a function to reverse the order of words in a given string. The function should return the reversed string. If the input string is empty, the function should return an empty string.
  
  assert reverse_words("Hello World")=="World Hello"
  assert reverse_words("Python is fun")=="fun is Python"
  assert reverse_words("SingleWord")=="SingleWord"
  assert reverse_words("")==""
output: def reverse_words(s):
      words = s.split()
      reversed_words = words[::-1]
      return ' '.join(reversed_words)