public static List<Integer> missingNumbers(List<Integer> arr, List<Integer> brr) {
        List<Integer> arrCopy = new ArrayList<>(arr);
        Set<Integer> missing = new TreeSet<>();
        for (Integer b: brr) {
            if (!arrCopy.contains(b)) {
                missing.add(b);
            } else { arrCopy.remove(b); }
        }
        return missing.stream().collect(Collectors.toList());
    }