title: More efficient than acc.reverse ::: b?
body: <p>I'm tail optimizing a recursive function. At the end, the result will be <code>acc.reverse ::: b</code>. This is O(n) because of <code>reverse</code> and <code>:::</code>. Is there a better performance way to combine the two lists? Thanks.</p>
  
  <p>Ex. Combine <code>List(3, 2, 1)</code> and  <code>List(4, 5, 6)</code> to <code>List(1, 2, 3, 4, 5, 6)</code></p>