stl merge

 std::vector<int> vec;
 std::vector<int> vec1;
 std::vector<int> vec2;

 vec1.push_back(1);
 vec1.push_back(3);
 vec1.push_back(5);

 vec2.push_back(2);
 vec2.push_back(4);
 vec2.push_back(6);

 vec.resize(6);

 std::merge(vec1.begin(), vec1.end(), vec2.begin(), vec2.end(), vec.begin());