public static List<Integer> countingSort(List<Integer> arr) {
        
        Map<Integer, Integer> countMap = new HashMap<>();
        
        for(int i=0; i< arr.size(); i++){
            int current = arr.get(i);
            if(countMap.containsKey(current)){
                int count = countMap.get(current);
                countMap.put(current, count+1);
            }else{
                countMap.put(current, 1);
            }
        }
        
        List<Integer> list = new ArrayList<>(100);
        for(int i=0; i<100; i++){
            if(countMap.containsKey((i))){
                list.add(countMap.get(i));
            }else{
                list.add(0);
            }
        }
        
        return list;
    }