int qs_find_pivot(Student* a, int i, int j)
{
    for (int t = i + 1; t <= j; t++)
    {
        if (a[i] < a[t]) return t;
        if (a[i] > a[t]) return i;
    }
    return -1;
}

int qs_partition(Student* a, int i, int j, int ip)
{
    int l = i, r = j;
    Student t = a[ip];

    do
    {
        while (a[l] < t) l++;
        while (a[r] >= t) r--;
        if (l < r) swap(a[r], a[l]);
    } while (l <= r);
    return l;
}

void QuickSort(Student * a, int n)
{
    int ip, k, i, j;
    queue<int> s;
    s.push(0);
    s.push(n - 1);

    while (!s.empty())
    {
        i = s.front();
        s.pop();
        j = s.front();
        s.pop();
        ip = qs_find_pivot(a, i, j);
        if (ip < 0) continue;

        k = qs_partition(a, i, j, ip);
        s.push(i);
        s.push(k - 1);
        s.push(k);
        s.push(j);
    }
}