栈的应用：在四则运算中序式转后续式的基础上，利用栈的帮助完成后序式的计算，并输出结果。(Application of stack: in the four operations, in order to follow up the follow-up, based on the help of the stack to complete the sequent calculation, and output the results.)

简易计算器，实现四则运算功能，仅供参考，欢迎补充(Simple calculator, the realization of the four arithmetic functions, for reference only.)