每一时刻，列车可以从入口车站或直接从入口进入出口，再或者从车站进入出口。即每一时刻可以有一辆车沿着箭头a或b或c的方向行驶。现在有一些车在入口处等待，它们要通过上述的方式从出口出来，现在你要统计出所有可能的出站序列。例如当入口是ABC时，可能的出站序列有ABC,ACB,BAC,BCA,CBA。(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.)