<p>You are given a doubly linked list, which contains nodes that have a next pointer, a previous pointer, and an additional <strong>child pointer</strong>. This child pointer may or may not point to a separate doubly linked list, also containing these special nodes. These child lists may have one or more children of their own, and so on, to produce a <strong>multilevel data structure</strong> as shown in the example below.</p>
<p>Given the <code>head</code> of the first level of the list, <strong>flatten</strong> the list so that all the nodes appear in a single-level, doubly linked list. Let <code>curr</code> be a node with a child list. The nodes in the child list should appear <strong>after</strong><code>curr</code> and <strong>before</strong><code>curr.next</code> in the flattened list.</p>
<p>Return <em>the </em><code>head</code><em> of the flattened list. The nodes in the list must have <strong>all</strong> of their child pointers set to </em><code>null</code>.</p>
<p>The serialization of each level is as follows:</p>
<pre>
[1,2,3,4,5,6,null]
[7,8,9,10,null]
[11,12,null]
</pre>
<p>To serialize all levels together, we will add nulls in each level to signify no node connects to the upper node of the previous level. The serialization becomes:</p>
<pre>
[1, 2, 3, 4, 5, 6, null]
|
[null, null, 7, 8, 9, 10, null]
|
[ null, 11, 12, null]
</pre>
<p>Merging the serialization of each level and removing trailing nulls we obtain:</p>
