public static boolean isValidBST2Helper(TreeNode root, int low, int high) {
		if (root == null) {
			return true;
		}
		if (root.val > low && root.val < high) {
			return isValidBST2Helper(root.left, low, root.val)
					&& isValidBST2Helper(root.right, root.val, high);
		} else {
			return false;
		}
	}