c 2b 2b vector extend vector

1std::vector<int> a = {1, 2};
2std::vector<int> b = {3, 4, 5};
3a.insert(a.end(), b.begin(), b.end()); // a = {1, 2, 3, 4, 5}

1std::vector<int> vec = {1, 2, 3};
2vec.resize(2); // {1, 2}
3vec.resize(4); // {1, 2, 0, 0,}
4vec.resize(6, 9);  // {1, 2, 0, 0, 9, 9}