Level order traversal

class Solution {
    public ArrayList<ArrayList<Integer>> levelOrder(Node root) {
        ArrayList<ArrayList<Integer>> list = new ArrayList<>();
        if (root == null)
            return list;
            
        Queue<Node> q = new LinkedList<>();
        q.add(root);

        while (!q.isEmpty()) {
            int size = q.size();
            ArrayList<Integer> ll = new ArrayList<>();
            for (int i = 0; i < size; i++) {
                Node curr = q.remove();
                ll.add(curr.data);
                if (curr.left != null)
                    q.add(curr.left);
                if (curr.right != null)
                    q.add(curr.right);
            }
            list.add(ll);
        }
        return list;
    }
}