STL的stack，省去开数组的麻烦了。
stack不能使用迭代器啊……有点麻烦。还好自动重载了=……
题目中

BACK: Push the current page on the top of the forward stack. Pop the page from the top of the backward stack, making it the new current page. If the backward stack is empty, the command is ignored. 
FORWARD: Push the current page on the top of the backward stack. Pop the page from the top of the forward stack, making it the new current page. If the forward stack is empty, the command is ignored. 

有点误导性~嘿嘿，阴险啊~