qsort: version 2

/*qsort: sort v[left] ... v[right] into increasing order */
void qsort(void *v[], int left, int right,
            int (*comp)(void *, void *)){
     int i, last;
     void swap(void *v[], int ,int);

     if (left >= right)  /*do nothing if array contains */
         return;         /* fewer than two elements */
     swap(v, left, (left + right)/2);
     last = left;
     for (i = left + 1; i<= right;i++)
         if ((*comp)(v[i], v[left]) < 0)
             swap(v, ++last, i);
     swap(v, left, last);
     qsort(v, left, last - 1, comp);
     qsort(v, last + 1, right, comp);
   
}