Balanced Binary Tree
<p>Given a binary tree, determine if it is <span data-keyword="height-balanced"><strong>height-balanced</strong></span>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/0100-0199/0110.Balanced%20Binary%20Tree/images/balance_1.jpg" style="width: 342px; height: 221px;" />
<pre>
<strong>Input:</strong> root = [3,9,20,null,null,15,7]
<strong>Output:</strong> true
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/0100-0199/0110.Balanced%20Binary%20Tree/images/balance_2.jpg" style="width: 452px; height: 301px;" />
<pre>
<strong>Input:</strong> root = [1,2,2,3,3,null,null,4,4]
<strong>Output:</strong> false
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> root = []
<strong>Output:</strong> true
</pre>

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

<ul>
	<li>The number of nodes in the tree is in the range <code>[0, 5000]</code>.</li>
	<li><code>-10<sup>4</sup> &lt;= Node.val &lt;= 10<sup>4</sup></code></li>
</ul>
Tree; Depth-First Search; Binary Tree
/**
 * Definition for a binary tree node.
 * class TreeNode {
 *     val: number
 *     left: TreeNode | null
 *     right: TreeNode | null
 *     constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
 *         this.val = (val===undefined ? 0 : val)
 *         this.left = (left===undefined ? null : left)
 *         this.right = (right===undefined ? null : right)
 *     }
 * }
 */

function isBalanced(root: TreeNode | null): boolean {
    const dfs = (root: TreeNode | null) => {
        if (root == null) {
            return 0;
        }
        const left = dfs(root.left);
        const right = dfs(root.right);
        if (left === -1 || right === -1 || Math.abs(left - right) > 1) {
            return -1;
        }
        return 1 + Math.max(left, right);
    };
    return dfs(root) > -1;
}
