说明：  表达式求值，逆波兰表达式算法，支持任何位数值运算，运算符支持+-*/()，其它运算符请自行扩展，代码比较松耦合可扩展性好
(Expression evaluation, inverse Polish expression algorithm, support any numeric operation, operator supports +-* / (), other operators please expand yourself, the code is loosely coupled, and the scalability)