实现计算器类以及相关函数函数。
2、完善计算器类的检查函数，增加对输入的合法性检查，包括滤掉所有非法输入及处理左右括号不配对的输入。
3、编制应用程序测试这个计算器(The calculator class and the related function function are implemented.
2, improve the checking function of the calculator class, increase the validity of the input, including filtering out all the illegal input and processing the unmatched input of the left and right brackets.
3. Write the application test the calculator)