如果值等于键，则合并映射中的条目

Question

I have a std::map with the following values:

2 31
4 36
5 29
6 24
24 49
25 83
29 63
36 42
42 79

Now I want to "merge" values if a key exists for a value. So the desired output (data structure is irrelevant) would be:

2 31
4 36 42 79
5 29 63
6 24 49
25 83

I have tried iterating through the map and using std::find for every value. But I am having problems with vector sizes greater than 3 and it seems to be very slow for large maps. Here is a small example not giving exactly the desired output:

int main(int argc, char** argv)
{   
    std::map<int, int> my_map = { {2, 31}, {4, 36}, {5, 29}, {6, 24}, {24, 49}, {25, 83}, {29, 63}, {36, 42}, {42, 79} };

    std::vector<int> temp_vec;
    std::vector<std::vector<int>> destination_vec;

    for (auto it = my_map.begin(); it != my_map.end(); ++it) {

        std::map<int, int>::iterator map_iterator = my_map.find(it->second);
        if (map_iterator == my_map.end()) {
            temp_vec.push_back(it->first);
            temp_vec.push_back(it->second);
        }
        else {
            temp_vec.push_back(it->first);
            temp_vec.push_back(it->second);
            temp_vec.push_back(map_iterator->second);
            // I stopped here because I could try another if loop here or a while loop for the whole process but it seems very inefficient
        }
        destination_vec.push_back(temp_vec);
        temp_vec.clear();
    }
}   

Answer 1

Assuming your map cannot link smaller values (as {{1, 42},{2, 1}} ).

You might use:

std::map<int, std::vector<int>> foo(std::map<int, int> m)
{
    std::map<int, std::vector<int>> res;

    while (!m.empty())
    {
        auto it = m.begin();
        const auto key = it->first;
        auto& v = res[key];

        while (it != m.end()) {
            auto value = it->second;
            v.push_back(value);
            m.erase(it);
            it = m.find(value);
        }
    }
    return res;
}

Demo

complexity is O(n log n) .