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 depth of the two subtrees of_every_node never differ by more than 1.

Example 1:

Given the following tree[3,9,20,null,null,15,7]:

    3
   / \
  9  20
    /  \
   15   7

Return true.

Example 2:

Given the following tree[1,2,2,3,3,null,null,4,4]:

       1
      / \
     2   2
    / \
   3   3
  / \
 4   4

Return false.

tag: binary tree

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    private class MyTree{
        boolean balanced;
        int height;

        public MyTree(boolean balanced, int height){
            this.balanced = balanced;
            this.height = height;
        }
    }

    public boolean isBalanced(TreeNode root) {
        if (root == null) return true;
        return helper(root).balanced;
    }

    private MyTree helper(TreeNode root){
        if (root == null){
            return new MyTree(true, 0);
        }

        MyTree leftTree = helper(root.left);
        MyTree rightTree = helper(root.right);

        boolean balanced = leftTree.balanced && rightTree.balanced && Math.abs(leftTree.height - rightTree.height) <= 1;
        int height = Math.max(leftTree.height, rightTree.height) + 1;

        return new MyTree(balanced, height);
    }
}