// 递归
    public List<Integer> preorderTraversal1(TreeNode root) {
        List<Integer> list = new LinkedList<>();
        helper(root, list);
        return list;
    }

    private void helper(TreeNode node, List<Integer> list) {
        if (node == null) {
            return;
        }
        list.add(node.val);
        helper(node.left, list);
        helper(node.right, list);
    }

    // 迭代
    public List<Integer> preorderTraversal2(TreeNode root) {
        LinkedList<TreeNode> stack = new LinkedList<>();
        LinkedList<Integer> output = new LinkedList<>();
        if (root == null) {
            return output;
        }
        stack.add(root);
        TreeNode node = null;
        while (!stack.isEmpty()) {
            node = stack.pollLast();
            output.add(node.val);
            if (node.right != null) {
                stack.add(node.right);
            }
            if (node.left != null) {
                stack.add(node.left);
            }
        }
        return output;
    }