title: combine two list and sort recursively
body: <p>I'm trying to define a function named combine to take two lists as parameters and combine them into one sorted list recursively.</p>
  
  <pre><code>def combine (l1,l2):
      if l1 == []:
          return l2
      elif l2 == []:
          return l1
      elif l1 == [] and l2 == []:
          return []
      sort_l = []
      index = 0
      for num in l1:
          if num &lt;= l2[0]:
              sort_l.append(num)
              index += 1
          else:
              return combine(l2, l1[index:])
      return sort_l + l2
  </code></pre>
  
  <p>It gives me:</p>
  
  <pre><code>combine([1,3,5,7,9],[0,2,4,6,8]) -&gt; [8, 9] 
  </code></pre>
  
  <p>but should:</p>
  
  <pre><code>[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
  </code></pre>
  
  <p>Any advice???</p>