vector<Student> sortByPhoneDirectory(const vector<Student> &students)
{
    vector<Student> sortedStudents = students;

    sort(sortedStudents.begin(), sortedStudents.end(), [](Student &first, Student &second) {

        if (first.getLastName() != second.getLastName()) {
            return first.getLastName() < second.getLastName();
        }

        return first.getFirstName() < second.getFirstName();
    });

    return sortedStudents;
}