private boolean isSymmetricAux(TreeNode left, TreeNode right) {

        if (left == null && right == null) {
            return true;
        }
        if (left == null || right == null) {
            return false;
        }
        if (left.getVal() != right.getVal()) {
            return false;
        }

        return isSymmetricAux(left.getLeft(), right.getRight()) && isSymmetricAux(left.getRight(), right.getLeft());
    }