0332-reconstruct-itinerary.cpp

/*
    Given airline tickets, find valid itinerary (use all tickets once)
    Ex. tickets = [["MUC","LHR"],["JFK","MUC"],["SFO","SJC"],["LHR","SFO"]]
        output = ["JFK","MUC","LHR","SFO","SJC"]

    Greedy DFS, build route backwards when retreating, merge cycles into main path

    Time: O(E log (E / V)) -> E = # of flights, V = # of airports, sorting
    Space: O(V + E) -> store # of airports & # of flights in hash map
*/

class Solution {
public:
    vector<string> findItinerary(vector<vector<string>>& tickets) {
        unordered_map<string, multiset<string>> m;
        for (int i = 0; i < tickets.size(); i++) {
            m[tickets[i][0]].insert(tickets[i][1]);
        }
        
        vector<string> result;
        dfs(m, "JFK", result);
        reverse(result.begin(), result.end());
        return result;
    }
private:
    void dfs(unordered_map<string, multiset<string>>& m,
        string airport, vector<string>& result) {
        
        while (!m[airport].empty()) {
            string next = *m[airport].begin();
            m[airport].erase(m[airport].begin());
            dfs(m, next, result);
        }
        
        result.push_back(airport);
    }
};