Stack 基本使用方法：http://c.biancheng.net/view/478.html

为什么pop不返回元素：https://blog.csdn.net/kylehit/article/details/8733788?utm_medium=distribute.pc_relevant.none-task-blog-BlogCommendFromMachineLearnPai2-14.control&dist_request_id=&depth_1-utm_source=distribute.pc_relevant.none-task-blog-BlogCommendFromMachineLearnPai2-14.control

pop返回 void