void quicksort(int * a, int l, int r){    if (l < r)    {        int high = r;        int low = l;        int key = a[l];        while (low < high)        {            while(low < high && a[high] > key)            {                high --;            }            a[low] = a[high];            while (low < high && a[low] < key)            {                low ++;            }            a[high] = a[low];            quicksort(a, l, low - 1);            quicksort(a, low + 1, r);         }     }}