static ArrayList<Integer> merge_sort(ArrayList<Integer> arr) {
        // Write your code here.
       merge_sort_helper(arr, 0, arr.size()-1);
        return arr;
    }

    static  void merge_sort_helper(ArrayList<Integer> arr, int start, int end) {
        ArrayList<Integer> auxList = new ArrayList<>();
        // Base Case
        if(start == end) {
            return;
        }

        // Recursive case
        int mid = start + (end - start) /2;
        merge_sort_helper(arr, start, mid);
        merge_sort_helper(arr, mid+1, end);

        // Merge sub-list
        int i=start;
        int j=mid+1;

        while(i<=mid && j<=end) {
            if(arr.get(i) <= arr.get(j)) {
                auxList.add(arr.get(i));
                i++;
            } else {
                auxList.add(arr.get(j));
                j++;
            }
        }
        while(i<=mid) {
            auxList.add(arr.get(i));
             i++;
        }
        while(j<=end) {
            auxList.add(arr.get(j));
            j++;
        }

        for(int idx = start; idx <= end; idx++) {
            arr.set(idx, auxList.get(idx - start));
        }
    }