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