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.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func isBalanced(root *TreeNode) bool {
	var height func(*TreeNode) int
	height = func(root *TreeNode) int {
		if root == nil {
			return 0
		}
		l, r := height(root.Left), height(root.Right)
		if l == -1 || r == -1 || abs(l-r) > 1 {
			return -1
		}
		if l > r {
			return 1 + l
		}
		return 1 + r
	}
	return height(root) >= 0
}

func abs(x int) int {
	if x < 0 {
		return -x
	}
	return x
}