quicksort

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);
         }
     }
}