332.cpp

class Solution {
private:
    unordered_map<string, deque<string>> trips_;  
    vector<string> route_;
    
public:
    vector<string> findItinerary(vector<pair<string, string>> tickets) {
        route_.clear();
        trips_.clear();
        for(const auto& pair: tickets)
            trips_[pair.first].push_back(pair.second);
            
        for(auto& pair: trips_) {
            auto& dest = pair.second;
            std::sort(dest.begin(), dest.end());
        }
        const string kStart = "JFK";
        visit(kStart);
        return vector<string>(route_.crbegin(), route_.crend());
    }
    void  visit(const string& src) {
        auto& dests = trips_[src];
        while (!dests.empty()) {
            const string dest = dests.front();
            dests.pop_front();
            visit(dest);
        }
        route_.push_back(src);
    }
};