public static class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;

    public TreeNode(int val) {
        this.val = val;
    }
}

public static List<List<Integer>> bfs(TreeNode root) {
    List<List<Integer>> result = new ArrayList<>();
    Queue<TreeNode> q = new LinkedList<>();
    q.offer(root);

    while(!q.isEmpty()) {
        int lvl = q.size();
        List<Integer> lvlNodes = new ArrayList<>();

        for (int i = 0; i < lvl; i++) {
            TreeNode currentNode = q.poll();
            lvlNodes.add(currentNode.val);

            if (currentNode.left != null) q.offer(currentNode.left);
            if (currentNode.right != null) q.offer(currentNode.right);
        }

        result.add(lvlNodes);
    }

    return result;
}