// Divide Conquar
public ArrayList<Integer> preorderTraversal(TreeNode root) {
        ArrayList<Integer> res = new ArrayList<Integer>();
        if (root == null) {
            return res;
        }

        ArrayList<Integer> left = preorderTraversal(root.left);
        ArrayList<Integer> right = preorderTraversal(root.right);
        res.add(root.val);
        res.addAll(left);
        res.addAll(right);
        return res;
    }