UnderGround System

An underground railway system is keeping track of customer travel times between different stations. They are using this data to calculate the average time it takes to travel from one station to another.

Requirement

Implement the UndergroundSystem class:

• check_in(id, station_name, time)
  • A customer with a card id, checks in at the station station_name at the time. A customer can only be checked into one place at a time.
• check_out(id, station_name, time)
  • A customer with a card id, checks out from the station station_name at the time.
• get_average_time(start_station, end_station)
  • Returns the average time it takes to travel from start_station to end_station.

Notes

1. The average time is computed from all the previous traveling times from start_station to end_station that happened directly, meaning a check in at start_station followed by a check out from end_station.
2. The time it takes to travel from start_station to end_station may be different from the time it takes to travel from end_station to start_station.
3. There will be at least one customer that has traveled from start_station to end_station before get_average_time is called.
4. You may assume all calls to the check_in and check_out methods are consistent.
5. If a customer checks in at time t1 then checks out at time t2, then t1 < t2.
6. All events happen in chronological order.

Input

Commands file:

• check<in/out>,id,station_name,time
• get_average,start_station,end_station

Example

check_in,45,Leyton,3
check_in,32,Paradise,8
check_out,45,Waterloo,15
check_out,32,Cambridge,22
get_average,Paradise,Cambridge

Output

Paradise,Cambridge,14.0

Example 1

Input

check_in,45,Leyton,3
check_in,32,Paradise,8
check_in,27,Leyton,10
check_out,45,Waterloo,15
check_out,27,Waterloo,20
check_out,32,Cambridge,22
get_average,Paradise,Cambridge
get_average,Leyton,Waterloo
check_in,10,Leyton,24
get_average,Leyton,Waterloo
check_in,10,Waterloo,38
get_average,Leyton,Waterloo

Output

Paradise,Cambridge,14.0
Leyton,Waterloo,11.0
Leyton,Waterloo,12.0