public static void main(String[] args) {
        List<Long> deltList = Arrays.asList(100L, 50L, 49L, 120L, 160L, 250L, 100L);
        Map<String, Integer> rangeFreq = new HashMap<>();

        for (Long delt : deltList) {
            String range = numToRange(delt, 50);

            Integer deltFreq = rangeFreq.get(range);

            if (deltFreq == null) {
                deltFreq = 0;
            }

            deltFreq += 1;
            rangeFreq.put(range, deltFreq);
        }

        System.out.println(rangeFreq);

    }

    public static String numToRange(Long delt, int step) {
        int rangeNum = (int) (delt / (long) step);
        int lower = rangeNum * step;
        int upper = lower + step - 1;
        return "" + lower + "-" + upper;
    }