std::vector<int> intersection(const std::vector<int>& a, const std::vector<int>& b){
    std::vector<int> merged = std::vector<int>(a);

    for (auto&& i : b){
        merged.push_back(i);
    }

    qsort(&(*merged.begin()), merged.size(), sizeof(int), [](const void* a1, const void* a2)
    {
        void *a1num = const_cast<void*> (a1),
             *a2num = const_cast<void*> (a2);

        return (*reinterpret_cast<int*>(a1num) > *reinterpret_cast<int*>(a2num)) ? 1 : -1;
    });

    int last = merged[0];

    std::vector<int> intersection;

    for (auto i = merged.begin() + 1; i < merged.end(); i++)
    {
        if (last == *i)
        {
            intersection.push_back(last);
        }
        last = *i;
    }
    return intersection;
}