public List<List<Integer>> levelOrderBottom(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(0, new ArrayList<>());
res.get(res.size() - depth - 1).add(node.val);
getLevelOrderByDFS(node.left, depth + 1, res);
getLevelOrderByDFS(node.right, depth + 1, res);