public int getBalance(TreeNode root){
        if(root == null) return 0;

        int left = getBalance(root.left);
        if(left == -1) return -1;

        int right = getBalance(root.right);
        if(right == -1) return -1;

        if(Math.abs(left - right) > 1) return -1;
        return left > right ? left + 1 : right + 1;
    }