Balanced Binary Tree
Difficulty: Easy
Category: DSA
Topics: Tree, Depth-First Search, Binary Tree
Asked at: Amazon
Given a binary tree, determine if it is **height-balanced**.
**Example 1:**
**Input:** root = [3,9,20,null,null,15,7]
**Output:** true
```
**Example 2:**
**Input:** root = [1,2,2,3,3,null,null,4,4]
**Output:** false
```
**Example 3:**
**Input:** root = []
**Output:** true
```
**Constraints:**
- The number of nodes in the tree is in the range `[0, 5000]`.
- `-104 <= Node.val <= 104`