public int[] mergeHelper(int[][] arr, int begin, int end) {
        if (begin >= end) {
            return arr[begin];
        }
        int mid = begin + (end - begin) / 2;
        int[] left = mergeHelper(arr, begin, mid);
        int[] right = mergeHelper(arr, mid + 1, end);
        return merge(left, right);
    }