title: Shuffle list in certain order recursively
body: <p>I'm trying to shuffle lists recursively in a certain order. </p>
  
  <p>I have:</p>
  
  <pre><code>def shuffle(list1, list2):
      a = []
      if len(list1) == 0:
          a += list2
          return a
      if len(list2) == 0:
          a += list1
          return a
      else:
          a += [list1[0]]
          list1.pop(0)
          a += [list2[0]]
          list2.pop(0)
      return a += shuffle(list1, list2)
  </code></pre>