extend a vector c++ code example

Example 1: c++ vector resize

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

Example 2: c++ vector extend vector

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