Sort and Merge two list of lists
<p>I have two lists,</p>

<pre><code>[[1, 2], [4, 7], [11, 13], [15, 21]]

[[3, 4], [5,12], [23, 25]]
</code></pre>

<p>I want an output like this.</p>

<pre><code>[[1, 2], [3,13], [15, 21], [23, 25]]
</code></pre>

<p>Anybody can help me?</p>
