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<h2><a href="https://leetcode.com/problems/recover-a-tree-from-preorder-traversal/">1028. Recover a Tree From Preorder Traversal</a></h2><h3>Hard</h3><hr><div><p>We run a&nbsp;preorder&nbsp;depth-first search (DFS) on the <code>root</code> of a binary tree.</p>
<p>At each node in this traversal, we output <code>D</code> dashes (where <code>D</code> is the depth of this node), then we output the value of this node.&nbsp; If the depth of a node is <code>D</code>, the depth of its immediate child is <code>D + 1</code>.&nbsp; The depth of the <code>root</code> node is <code>0</code>.</p>
<p>If a node has only one child, that child is guaranteed to be <strong>the left child</strong>.</p>
<p>Given the output <code>traversal</code> of this traversal, recover the tree and return <em>its</em> <code>root</code>.</p>
<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2019/04/08/recover-a-tree-from-preorder-traversal.png" style="width: 320px; height: 200px;">
<pre><strong>Input:</strong> traversal = "1-2--3--4-5--6--7"
<strong>Output:</strong> [1,2,5,3,4,6,7]
</pre>
<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2019/04/11/screen-shot-2019-04-10-at-114101-pm.png" style="width: 256px; height: 250px;">
<pre><strong>Input:</strong> traversal = "1-2--3---4-5--6---7"
<strong>Output:</strong> [1,2,5,3,null,6,null,4,null,7]
</pre>
<p><strong class="example">Example 3:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2019/04/11/screen-shot-2019-04-10-at-114955-pm.png" style="width: 276px; height: 250px;">
<pre><strong>Input:</strong> traversal = "1-401--349---90--88"
<strong>Output:</strong> [1,401,null,349,88,90]
</pre>
<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>
<ul>
<li>The number of nodes in the original tree is in the range <code>[1, 1000]</code>.</li>
<li><code>1 &lt;= Node.val &lt;= 10<sup>9</sup></code></li>
</ul>
</div>