Instructions

Implement the UndergroundSystem class with the following methods:

• checkIn(int id, string stationName, int t)
  A customer with a card ID equal to id, checks in at the station stationName at time t.
  A customer can only be checked into one place at a time.

• checkOut(int id, string stationName, int t)
  A customer with a card ID equal to id, checks out from the station stationName at time t.

• getAverageTime(string startStation, string endStation)
  Returns the average time it takes to travel from startStation to endStation.

Attempt here.

Approach

Some assumptions made when implementing the class

• The average time is computed from all the previous traveling times from startStation to endStation that happened directly, meaning a check in at startStation followed by a check out from endStation.

• The time it takes to travel from startStation to endStation may be different from the time it takes to travel from endStation to startStation.

• There will be at least one customer that has traveled from startStation to endStation before getAverageTime is called.

Implementation

The Underground class which will have the three methods.
Attributes: a record of checkin timescheck_in and a record of the start and stop station and time taken between them. Use a dictionary to maintain these recordstravel.

class UndergroundSystem:

    def __init__(self):
        #dictionary of checkin stationName and time of checkin. 
        self.check_in = dict()
        #dict of start and stop station and time taken btwn them. 
        self.travel = dict()

• The checkIn method adds the check in stationName and the time. These are stored as a tuple.

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

The checkOut method checks if the given customer id is in the checkin dictionary and gets the start stationName. This is used to
create a tuple of start and end station for a given id.
We also determine the time taken to travel between the stations.

def checkOut(self, id: int, stationName: str, t: int) -> None:
        if id in self.check_in:
            startStation, t1 = self.check_in[id]

            # tuple of stations travelled
            stations = (startStation, stationName)

            #determine time taken btwn stations
            if stations in self.travel:
                self.travel[stations].append(t - t1)
            else:
                self.travel[stations] = [t - t1]
            #remove customer from checkin dictionary

            self.check_in.pop(id)

The getAverageTime method checks if the pair of startstation and end station are in the travel dictionary.
The average time is calculated by getting the total time divided by the number of times traveled between the two stations.

def getAverageTime(self, startStation, endStation):
        stations = (startStation, endStation)
        if stations in self.travel:
            time_taken_list = self.travel[stations]
            return sum(time_taken_list) / len(time_taken_list)

Here is the entire code.