List operator&(const List& firstList, const List& secondList)
{
    List resultList;
    List::Node* firstPtr = firstList.head_;
    List::Node* secondPtr = secondList.head_;
    while (firstPtr != nullptr)
    {
        while (secondPtr != nullptr)
        {
            if (firstPtr->value_ == secondPtr->value_)
            {
                resultList += firstPtr->value_;
                break;
            }
            secondPtr = secondPtr->next_;
        }
        firstPtr = firstPtr->next_;
        secondPtr = secondList.head_;
    }
    return resultList;
}