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"title": "Flatten a Multilevel Doubly Linked List",
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"content": "<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>\n\n<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>\n\n<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>\n\n<p> </p>\n<p><strong>Example 1:</strong></p>\n<img alt=\"\" src=\"https://assets.leetcode.com/uploads/2021/11/09/flatten11.jpg\" style=\"width: 700px; height: 339px;\" />\n<pre>\n<strong>Input:</strong> head = [1,2,3,4,5,6,null,null,null,7,8,9,10,null,null,11,12]\n<strong>Output:</strong> [1,2,3,7,8,11,12,9,10,4,5,6]\n<strong>Explanation:</strong> The multilevel linked list in the input is shown.\nAfter flattening the multilevel linked list it becomes:\n<img src=\"https://assets.leetcode.com/uploads/2021/11/09/flatten12.jpg\" style=\"width: 1000px; height: 69px;\" />\n</pre>\n\n<p><strong>Example 2:</strong></p>\n<img alt=\"\" src=\"https://assets.leetcode.com/uploads/2021/11/09/flatten2.1jpg\" style=\"width: 200px; height: 200px;\" />\n<pre>\n<strong>Input:</strong> head = [1,2,null,3]\n<strong>Output:</strong> [1,3,2]\n<strong>Explanation:</strong> The multilevel linked list in the input is shown.\nAfter flattening the multilevel linked list it becomes:\n<img src=\"https://assets.leetcode.com/uploads/2021/11/24/list.jpg\" style=\"width: 300px; height: 87px;\" />\n</pre>\n\n<p><strong>Example 3:</strong></p>\n\n<pre>\n<strong>Input:</strong> head = []\n<strong>Output:</strong> []\n<strong>Explanation:</strong> There could be empty list in the input.\n</pre>\n\n<p> </p>\n<p><strong>Constraints:</strong></p>\n\n<ul>\n\t<li>The number of Nodes will not exceed <code>1000</code>.</li>\n\t<li><code>1 <= Node.val <= 10<sup>5</sup></code></li>\n</ul>\n\n<p> </p>\n<p><strong>How the multilevel linked list is represented in test cases:</strong></p>\n\n<p>We use the multilevel linked list from <strong>Example 1</strong> above:</p>\n\n<pre>\n 1---2---3---4---5---6--NULL\n |\n 7---8---9---10--NULL\n |\n 11--12--NULL</pre>\n\n<p>The serialization of each level is as follows:</p>\n\n<pre>\n[1,2,3,4,5,6,null]\n[7,8,9,10,null]\n[11,12,null]\n</pre>\n\n<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>\n\n<pre>\n[1, 2, 3, 4, 5, 6, null]\n |\n[null, null, 7, 8, 9, 10, null]\n |\n[ null, 11, 12, null]\n</pre>\n\n<p>Merging the serialization of each level and removing trailing nulls we obtain:</p>\n\n<pre>\n[1,2,3,4,5,6,null,null,null,7,8,9,10,null,null,11,12]\n</pre>\n",
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