Practice/Ramp/Leetcode 1396. Design Underground System

Leetcode 1396. Design Underground System

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Problem

Design and implement a transit tracking system that monitors passenger movements through a transportation network. Your system needs to track when passengers enter and exit stations, then calculate the average travel time between any two stations.

The system should support three operations:

1. checkIn(passenger_id, station_name, timestamp): Record that a passenger with the given ID checked in at the specified station at the given time.
2. checkOut(passenger_id, station_name, timestamp): Record that a passenger with the given ID checked out at the specified station at the given time.
3. getAverageTime(start_station, end_station): Return the average travel time for all trips that went directly from start_station to end_station.

You can assume:

• A passenger can only be checked in at one station at a time
• Each passenger checks out before checking in again
• All queries to getAverageTime will have at least one completed trip for that route

Requirements

1. Implement the TransitTracker class with the three methods described above
2. Track active check-ins for passengers currently in transit
3. Maintain cumulative statistics (total time and trip count) for each station pair
4. Calculate averages efficiently without iterating through all past trips
5. Return the average as a floating-point number

Constraints

• 1 ≤ passenger_id, timestamp ≤ 10^6
• 1 ≤ station_name.length ≤ 10
• Station names consist of uppercase and lowercase English letters
• At most 2 × 10^4 total operations will be performed
• getAverageTime is only called when at least one trip exists for the route
• All checkIn/checkOut operations are valid (no invalid sequences)

Examples

Example 1:

` operations: ["checkIn", "checkOut", "getAverageTime"] params: [[45, "CentralStation", 3], [45, "WestTerminal", 21], ["CentralStation", "WestTerminal"]]

Output: 18.0

Explanation: Passenger 45 checks in at CentralStation at time 3 and checks out at WestTerminal at time 21. The trip duration is 21 - 3 = 18 minutes. Since this is the only trip on this route, the average is 18.0. `

Example 2:

` operations: ["checkIn", "checkIn", "checkOut", "checkOut", "getAverageTime"] params: [[10, "StationA", 5], [20, "StationA", 10], [10, "StationB", 15], [20, "StationB", 25], ["StationA", "StationB"]]

Output: 12.5

Explanation:

• Passenger 10: checks in at StationA (time 5), checks out at StationB (time 15). Duration = 10 minutes.
• Passenger 20: checks in at StationA (time 10), checks out at StationB (time 25). Duration = 15 minutes.
• Average time for StationA → StationB = (10 + 15) / 2 = 12.5 minutes. `

Example 3:

` operations: ["checkIn", "checkOut", "checkIn", "checkOut", "getAverageTime", "getAverageTime"] params: [[1, "X", 0], [1, "Y", 10], [1, "Y", 20], [1, "Z", 35], ["X", "Y"], ["Y", "Z"]]

Output: 10.0, 15.0

Explanation:

• Passenger 1 travels X → Y taking 10 minutes
• Passenger 1 later travels Y → Z taking 15 minutes
• These are tracked as separate routes with their respective averages `