C++中map按value排序

int main()
{
    map<int, int> iMap;
    iMap[1] = 20;
    iMap[2] = 10;
    iMap[5] = 30;
    iMap[4] = 0;
    for (auto it = iMap.begin(); it != iMap.end(); it++)
        cout << it->first << ':' << it->second << '\n';
    return 0;
}

int main()
{
    unordered_map<int, int> iMap;
    iMap[1] = 20;
    iMap[2] = 10;
    iMap[5] = 30;
    iMap[4] = 0;

    vector<pair<int, int>> vtMap;
    for (auto it = iMap.begin(); it != iMap.end(); it++)
        vtMap.push_back(make_pair(it->first, it->second));

    sort(vtMap.begin(), vtMap.end(), 
        [](const pair<int, int> &x, const pair<int, int> &y) -> int {
        return x.second < y.second;
    });

    for (auto it = vtMap.begin(); it != vtMap.end(); it++)
        cout << it->first << ':' << it->second << '\n';
    return 0;
}