Maximum Depth of Binary Tree
<p>Given the <code>root</code> of a binary tree, return <em>its maximum depth</em>.</p>

<p>A binary tree&#39;s <strong>maximum depth</strong>&nbsp;is the number of nodes along the longest path from the root node down to the farthest leaf node.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/0100-0199/0104.Maximum%20Depth%20of%20Binary%20Tree/images/tmp-tree.jpg" style="width: 400px; height: 277px;" />
<pre>
<strong>Input:</strong> root = [3,9,20,null,null,15,7]
<strong>Output:</strong> 3
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> root = [1,null,2]
<strong>Output:</strong> 2
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li>The number of nodes in the tree is in the range <code>[0, 10<sup>4</sup>]</code>.</li>
	<li><code>-100 &lt;= Node.val &lt;= 100</code></li>
</ul>
Tree; Depth-First Search; Breadth-First Search; Binary Tree
/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number}
 */
var maxDepth = function (root) {
    if (!root) return 0;
    const l = maxDepth(root.left);
    const r = maxDepth(root.right);
    return 1 + Math.max(l, r);
};
