private static void qs(int items[], int left, int right)
  {
    int i, j;
    int x, y;
    i = left; j = right;
    x = items[(left+right)/2];
    do {
      while((items[i] < x) && (i < right)) i++;
      while((x < items[j]) && (j > left)) j--;
      if(i <= j) {
        y = items[i];
        items[i] = items[j];
        items[j] = y;
        i++; j--;
      }
    } while(i <= j);
    if(left < j) qs(items, left, j);
    if(i < right) qs(items, i, right);
  }