c++ - How to collect the same type of Map's value -

we have 1 map key , value both int type. have search particular value in map , collect key in 1 vector. code snapshot like

map<int,int>m; map<int,int>::iterator itr; vector<int> v; m.insert(make_pair<int,int>(1,2)); m.insert(make_pair<int,int>(2,2)); m.insert(make_pair<int,int>(3,2)); m.insert(make_pair<int,int>(4,4)); m.insert(make_pair<int,int>(5,5)); 

and current code like:

for ( itr = m.begin(); itr != m.end(); ++itr ) {     if ((*itr).second == 2 )     v.push_back((*itr).first ) } 

we optimize it. how can stl algorithm.

it seems me going wrong way, want multimap.

std::multimap<int,int> m; std::vector<int> v; m.insert(std::make_pair<int,int>(2,1)); m.insert(std::make_pair<int,int>(2,2)); m.insert(std::make_pair<int,int>(2,3)); m.insert(std::make_pair<int,int>(4,4)); m.insert(std::make_pair<int,int>(5,5));  typedef std::multimap<int,int>::iterator iterator; std::pair<iterator, iterator> bounds = m.equal_range(2); for(iterator = bounds.first; != bounds.second; ++it)    v.push_back(it->second);