public List<List<Integer>> levelOrder(TreeNode root) {
if (root == null) return new ArrayList<>();
List<List<Integer>> res = new ArrayList<>();
Deque<TreeNode> queue = new LinkedList<>();
queue.offer(root);
while (!queue.isEmpty()){
int size = queue.size();
ArrayList<Integer> list = new ArrayList<>();
for (int i = 0; i < size; i++) {
TreeNode node = queue.pop();
list.add(node.val);
if (node.left != null) queue.offer(node.left);
if (node.right != null) queue.offer(node.right);
}
res.add(list);
}
return res;
}