希尔排序

void shell_sort()
{
    for (int gap = n >> 1; gap; gap >>= 1)
    {
        for (int i = gap; i < n; i ++ )
        {
            int x = a[i];
            int j;
            for (j = i; j >= gap && a[j-gap] > x; j -= gap)
                a[j] = a[j-gap];
            a[j] = x;
        }
    }
}