Given a binary tree, determine if it is height-balanced.

For this problem, a height-balanced binary tree is defined as:

> a binary tree in which the left and right subtrees of every node differ in height by no more than 1.

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`