北航oo第二次作业，做一个傻瓜电梯。只能单部电梯上下楼层不能捎带(Beihang OO second operation, make a fool elevator. Only one elevator can go up and down the stairs without carrying it.)

北航第三次oo作业，在原来傻瓜电梯的基础上增加捎带功能(Beihang third OO operation, add piggyback function)