public static String[] sort(String[] a, int W) {
        int N = a.length;
        int R = 256;   // extend ASCII alphabet size
        String[] aux = new String[N];

        for (int d = W-1; d >= 0; d--) {
            // sort by key-indexed counting on dth character

            // compute frequency counts
            int[] count = new int[R+1];
            for (int i = 0; i < N; i++)
                count[a[i].charAt(d) + 1]++;

            // compute cumulates
            for (int r = 0; r < R; r++)
                count[r+1] += count[r];

            // move data
            for (int i = 0; i < N; i++)
                aux[count[a[i].charAt(d)]++] = a[i];

            // copy back
            for (int i = 0; i < N; i++)
                a[i] = aux[i];
        }
        return a;
    }