code: private static void reverse(final byte[] arr, final int len) {
          for (int l = 0, r = 0 + len - 1; l < r; l++, r--) {
              final byte tmp = arr[l];
              arr[l] = arr[r];
              arr[r] = tmp;
          }
      }
docs: Reverses the order of all elements in the given interval.
  
  @param arr array
  @param len length of the interval