facebook-problem-c++

#include <iostream>
#include <algorithm>
#include <utility>
#include <map>
#include <list>

#define debug_print false   // pretty print things

bool pp(std::list<std::string> &to_print) {
    std::cout << "[ ";
    for( auto it = to_print.begin(); it != to_print.end(); it++ ) {
        std::cout << "'" << *it << "' ";
    }
    std::cout << "]\n";
    return true;
}

int main(void)
{
    int num_of_trips;
    std::map<std::string, std::list<std::string> > trips;
    // map with key -> list of strings

    std::cout << "Please, enter the number of trips you will provide: ";
    std::cin >> num_of_trips;

    // loop to insert all the entries
    for(int i = 0; i < num_of_trips; i++)
    {
        std::string city_a, city_b;
        std::cin >> city_a >> city_b;

        auto search_key = trips.find(city_a);
        if(search_key != trips.end()) {
            // found the key, insert on ordered position on the key
            search_key->second.insert(
                    std::lower_bound(
                        search_key->second.begin(),
                        search_key->second.end(),
                        city_b
                        ),
                    city_b
                    );
        } else {
            // didn't found the key, insert a new entry on map
            trips.insert(
                    std::pair<std::string, std::list<std::string> > (
                        city_a,
                        {city_b}
                        )
                    );
        }
    }

    std::list<std::string> itin;
    std::string actual_city;

    std::cout << "Please, enter the first city: ";
    std::cin >> actual_city;

    auto actual_it = trips.find(actual_city);

    // sanity check
    if( actual_it == trips.end() ) {
        std::cout << "City doesn't exist on the itinerary!\nExiting...\n";
        return 1;
    }

    std::string actual_city_name = actual_it->first;

    // populate itin with the actual itinerary
    while(true) {
        actual_it = trips.find(actual_city_name);
        if( actual_it != trips.end() ) {
            // exists
            itin.push_back(actual_it->first);
            if( actual_it->second.size() > 0 ) {
                actual_city_name = actual_it->second.front();
                actual_it->second.pop_front();
            } else {
                break;
            }
        } else {
            itin.push_back(actual_city_name);
            break;
        }
    }

    if( itin.size() != num_of_trips + 1 ){
        std::cout << "null" << std::endl;
        return 1;
    }

    pp(itin);
    return 0;
}