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