Description: At any time, the train can enter the exit from the entry station or directly from the entry, or from the station to the exit. That is to say, at any time, a car can drive along the direction of arrow a or B or C. Now there are some cars waiting at the entrance. They will exit from the above way. Now you need to count all the possible outgoing sequences. For example, when the entry is ABC, the possible outgoing sequences are ABC, ACB, BAC, BCA and CBA.
Station.cpp, 2487 , 2018-01-05