# Time: O(N)
# Space:   O(P)       P is num of passengers who checkin at same time worst case
#        + O(S^2)     S is num of stations, pair up every station so S^2 permutation

class UndergroundSystem:

    def __init__(self):
        self.checkins = {}
        self.totals = {}

    def checkIn(self, id: int, stationName: str, t: int) -> None:
        self.checkins[id] = (stationName, t)

    def checkOut(self, id: int, endStation: str, end_t: int) -> None:
        startStation, start_t = self.checkins[id]

        key = (startStation, endStation)
        duration = end_t - start_t
        
        curr_total = self.totals.get(key, (0, 0))
        
        # Increment total and count of journeys
        self.totals[key] = (curr_total[0] + duration, curr_total[1] + 1)
        
        # Journey is over for this passenger, we can remove it since we
        # tallied the data
        del self.checkins[id]
        

    def getAverageTime(self, startStation: str, endStation: str) -> float:
        total_duration, num_trips = self.totals[(startStation, endStation)]
        
        return total_duration / num_trips


# Your UndergroundSystem object will be instantiated and called as such:
# obj = UndergroundSystem()
# obj.checkIn(id,stationName,t)
# obj.checkOut(id,stationName,t)
# param_3 = obj.getAverageTime(startStation,endStation)