public List<List<Integer>> levelOrder(TreeNode root) {
List<List<Integer>> res = new ArrayList<>();
getLevelOrderByDFS(root, 0, res);
public void getLevelOrderByDFS(TreeNode node, int depth, List<List<Integer>> res) {
if (res.size() == depth) {
res.add(new ArrayList<>());
res.get(depth).add(node.val);
getLevelOrderByDFS(node.left, depth + 1, res);
getLevelOrderByDFS(node.right, depth + 1, res);