template<typename InputIt> void insert(InputIt first, InputIt last) {
    for (auto it=first; it != last; ++it) {
        size_type idx = hashIndex(*it);
        if(!(table[idx]->findElement(*it))) {
              insertElementInTable(*it);
        }
     }
}

size_type hashIndex(const_reference key) const {
    size_type idx = hasher{}(key) % (2^roundNum); //roundNum is initialized with 1
    size_type d{roundNum};

    if(idx < nextToSplit) {
        ++d;
        idx = hasher{}(key) % (2^d);
    }

    return idx;
}