vector的 emplace 和 insert

    vector<int> first;//Size()==2
    first.push_back(1);
    first.push_back(2);
    //first.insert(2);

    vector<int>second;//Size()==3 + assign??
    second.push_back(3);
    second.push_back(4);
    second.push_back(5);
    vector<int>::iterator it = first.begin();
    //second.assign(it,first.end());
    second.insert(second.end(),first.begin(),first.end());


    printf("first.size=%d\n", first.size());
    printf("second.size=%d\n", second.size());
    for (it = second.begin(); it != second.end(); it++)
        printf("second[]=%d\n", *it);