給定一個二叉樹，判斷它是否是高度平衡的二叉樹。

本題中，一棵高度平衡二叉樹定義為：

一個二叉樹每個節點的左右兩個子樹的高度差的絕對值不超過 1 。

來源：力扣（LeetCode）
連結：https://leetcode-cn.com/problems/balanced-binary-tree
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class Solution {

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

        Helper left = solve(root.left);
        Helper right = solve(root.right);

        return new Helper(left.isBalanced && right.isBalanced && Math.abs(left.height - right.height) <= 1,
                Math.max(left.height, right.height) + 1);

    }


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

        return solve(root).isBalanced;
    }
}

class Helper {
    boolean isBalanced;
    int height;

    public Helper(boolean isBalanced, int height) {
        this.isBalanced = isBalanced;
        this.height = height;
    }
}

class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;

    TreeNode() {
    }

    TreeNode(int val) {
        this.val = val;
    }

    TreeNode(int val, TreeNode left, TreeNode right) {
        this.val = val;
        this.left = left;
        this.right = right;
    }
}