void set_tree () {
        cout << "Введите количество элементов в дереве: ";
        int amount_of_elements;
        do {cin >> amount_of_elements; } while (amount_of_elements < 1);
        cout << "Введите " << amount_of_elements << " строки: " << endl;
        for (int i = 0; i < amount_of_elements; i ++) {
            cout << "  " << i + 1 << ": " ;
            string str; cin >> str;
            this->insert(str);
        }
    }