https://leetcode.com/problems/balanced-binary-tree/description/
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
The number of nodes in the tree is in the range [0, 5000].
-10^4 <= Node.val <= 10^4