Recover a Tree From Preorder Traversal
<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://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/1000-1099/1028.Recover%20a%20Tree%20From%20Preorder%20Traversal/images/recover_tree_ex1.png" style="width: 423px; height: 200px;" />
<pre>
<strong>Input:</strong> traversal = &quot;1-2--3--4-5--6--7&quot;
<strong>Output:</strong> [1,2,5,3,4,6,7]
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/1000-1099/1028.Recover%20a%20Tree%20From%20Preorder%20Traversal/images/recover_tree_ex2.png" style="width: 432px; height: 250px;" />
<pre>
<strong>Input:</strong> traversal = &quot;1-2--3---4-5--6---7&quot;
<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://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/1000-1099/1028.Recover%20a%20Tree%20From%20Preorder%20Traversal/images/recover_tree_ex3.png" style="width: 305px; height: 250px;" />
<pre>
<strong>Input:</strong> traversal = &quot;1-401--349---90--88&quot;
<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>
Tree; Depth-First Search; String; Binary Tree
function recoverFromPreorder(traversal: string): TreeNode | null {
    const stack: TreeNode[] = [];
    let i = 0;

    while (i < traversal.length) {
        let depth = 0;
        while (i < traversal.length && traversal[i] === '-') {
            depth++;
            i++;
        }

        let num = 0;
        while (i < traversal.length && !Number.isNaN(+traversal[i])) {
            num = num * 10 + +traversal[i];
            i++;
        }

        // Create the new node
        const newNode = new TreeNode(num);

        while (stack.length > depth) {
            stack.pop();
        }

        if (stack.length > 0) {
            const i = stack.length - 1;
            if (stack[i].left === null) {
                stack[i].left = newNode;
            } else {
                stack[i].right = newNode;
            }
        }

        stack.push(newNode);
    }

    return stack.length ? stack[0] : null;
}
