@Override
    public Map<Integer, List<E>> nodesByLevel() {
        if (root == null) {
            return null;
        }
        int level = 0;
        Map<Integer, List<E>> result = new TreeMap<>();
        processBstByLevel(root, result, 0);
        return result;
    }

    private void processBstByLevel(Node<E> node, Map<Integer, List<E>> result, int level) {
        LinkedList<Node<E>> currentLevel = new LinkedList<>();
        LinkedList<Node<E>> nextLevel = new LinkedList<>();
        LinkedList<E> elements = new LinkedList<>();

        currentLevel.add(node);
        while (!currentLevel.isEmpty()) {
            Node<E> nParent = currentLevel.remove();
            elements.add(nParent.getElement());

            if (nParent.getLeft() != null) {
                nextLevel.add(nParent.getLeft());
            }
            if (nParent.getRight() != null) {
                nextLevel.add(nParent.getRight());
            }

            if (currentLevel.isEmpty()) {
                result.put(level, new LinkedList<>(elements));
                elements.clear();
                level++;
                currentLevel.addAll(nextLevel);
                nextLevel.clear();
            }
        }
    }