LinkedList of Nodes at each level - optimalization

Question

That's already known problem: to return an array of linked lists that contain values of elements at each level. Eg for tree with depth n there should be n linked lists.

I wrote the solution:

public ArrayList<LinkedList<Integer>> levelLists() {

    class NodeLevel {
      public BinaryTreeNode node;
      public int level;

      public NodeLevel(BinaryTreeNode n, int l) {
        this.node = n;
        this.level = l;
      }
    }

    ArrayList<LinkedList<Integer>> levels = new ArrayList<LinkedList<Integer>>();
    Queue<NodeLevel> queue = new LinkedList<NodeLevel>();

    int currentLevel = -1;

    queue.add(new NodeLevel(root, 0));

    while (!queue.isEmpty()) {
      NodeLevel current = queue.poll();

      if (current.level != currentLevel) {
        levels.add(new LinkedList<Integer>());
        currentLevel++;
      }

      levels.get(levels.size() - 1).add(current.node.value);

      if (current.node.getLeftChild() != null) {
        queue.add(new NodeLevel(current.node.getLeftChild(), currentLevel + 1));
      }
      if (current.node.getRightChild() != null) {
        queue
            .add(new NodeLevel(current.node.getRightChild(), currentLevel + 1));
      }
    }

    return levels;
  }

But I am sure it's very space inefficient. I am quite sure it can be done without remembering level for each node (and so without this whole additional class NodeLevel). I was thinking about just adding null node to the queue at the end of each level, but still I don't know how to indicate end of the level without this whole structure. Can anyone give me a hint?