https://leetcode.com/problems/balanced-binary-tree/

Given a binary tree, determine if it is height-balanced (A height-balanced binary tree is a binary tree in which the depth of the two subtrees of every node never differs by more than one.).

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