Atlassian first round - rejected

Asked to implement following class:

//implement class to return most popular content ID  
class MostPopular {  
    void increasePopularity(int contentId);  
    int mostPopular();  
    void decreasePopularity(int contentId);  
}  

I wrote following solution:
Basically maintaining id_to_popularity and popularity_to_id maps. As there can be multiple IDs with same popularity so value for pop_to_id map is a vector of int. Best case all the 3 functions are o(1).

Code compiled and produced correct output. Don't know why I was rejected.

#include<iostream>  
#include<vector>  
#include<unordered_map>  
#include<set>  
  
using namespace std;  
  
//implement class to return most popular content ID  
class MostPopular {  
    unordered_map<int, int> id_to_pop;  
    unordered_map<int, set<int>> pop_to_id;  
    int max_pop;  
public:  
    MostPopular() {  
        max_pop = INT_MIN;  
    }  
    void increasePopularity(int contentId);  
  
    int mostPopular();  
  
    void decreasePopularity(int contentId);  
};  
  
void MostPopular::increasePopularity(int contentId) {  
  
    if (id_to_pop.find(contentId) == id_to_pop.end()) {  
        id_to_pop[contentId] = 1;  
        pop_to_id[1].insert(contentId);  
        //update max_pop  
        if (max_pop < 1) {  
            max_pop = 1;  
        }  
    } else {  
        int initial_pop = id_to_pop[contentId];  
        id_to_pop[contentId]++;  
        auto collection = pop_to_id[initial_pop];  
        collection.erase(contentId);  
          
        auto new_collection = pop_to_id[initial_pop+1];  
        new_collection.insert(contentId);  
        pop_to_id[initial_pop+1] = new_collection;  
        if (max_pop < initial_pop+1) {  
            max_pop = initial_pop+1;  
        }  
    }  
}  
  
int MostPopular::mostPopular() {  
    int ret = max_pop <= 0? -1: max_pop;  
    int ret_id = -1;  
    auto collection = pop_to_id[ret];  
    cout<<"New call to mostPopular"<<endl;  
    /*for(auto i = collection.begin(); i != collection.end(); i++) {  
        ret_id = *i;  
        cout<<*i<<endl;  
    }*/  
    //cout<<ret<<endl;  
    return ret;  
}  
  
void MostPopular::decreasePopularity(int contentId) {  
    if (id_to_pop.find(contentId) == id_to_pop.end()) {  
        id_to_pop[contentId] = -1;  
        pop_to_id[-1].insert(contentId);  
        //update max_pop  
        if (max_pop < -1) {  
            max_pop = -1;  
        }  
    } else {  
        int initial_pop = id_to_pop[contentId];  
        id_to_pop[contentId]--;  
        auto collection = pop_to_id[initial_pop];  
        collection.erase(contentId);  
          
        auto new_collection = pop_to_id[initial_pop-1];  
        new_collection.insert(contentId);  
          
        if (collection.size() == 0 && max_pop == initial_pop) {  
            max_pop = initial_pop-1;  
        }  
    }  
}  
//id->pop unordered_map<int, int>  
//pop->id unordered_map<int, vector<int>> //vector<int> which is a heap  
  
int main() {  
    //cout<<"Hello World!";  
    //return 0;  
    MostPopular popularityTracker;      
    popularityTracker.increasePopularity(7);  
  
  popularityTracker.increasePopularity(7);  
  
  popularityTracker.increasePopularity(8);  
  
  popularityTracker.mostPopular();        // returns 7  
  
  popularityTracker.increasePopularity(8);  
  
  popularityTracker.increasePopularity(8);  
  
  popularityTracker.mostPopular();        // returns 8  
  
  popularityTracker.decreasePopularity(8);  
  
  popularityTracker.decreasePopularity(8);  
  
  popularityTracker.mostPopular();        // returns 7  
  
  popularityTracker.decreasePopularity(7);  
  
  popularityTracker.decreasePopularity(7);  
  
  popularityTracker.decreasePopularity(8);  
  popularityTracker.mostPopular();  
      
}