private radix sort()

/* Digit-wise radix sort logic */
private static void sort(int[] elements, int exp) {
    int[] aux = new int[elements.length];                   // 1
    int[] frequency = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0};       // 2

    for (int i = 0; i < elements.length; i++) {             // 3
        int digit = (elements[i] / exp) % 10;               // 3
        frequency[digit]++;                                 // 3
    }

    for (int i = 1; i < 10; i++) {                          // 4
        frequency[i] += frequency[i - 1];                   // 4
    }

    for (int i = elements.length - 1; i >= 0; i--) {        // 5
        int digit = (elements[i] / exp) % 10;               // 5
        aux[frequency[digit] - 1] = elements[i];            // 5
        frequency[digit]--;                                 // 5
    }

    for (int i= 0; i < elements.length; i++) {              // 6
        elements[i] = aux[i];                               // 6
    }
}