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: 3 9 20 null null 15 7 Output: true
Example 2:
Input: 1 2 2 3 3 null null 4 4 Output: false
Constraints:
- The number of nodes in the tree is in the range [0, 5000]
- -10⁴ ≤ Node.val ≤ 10⁴
Input format: BFS level-order, space-separated, null for missing nodes.
Output format: true or false.