表达式计算是实现程序设计语言的基本问题之一，它的实现是栈的应用的一个典型例子。设计一个程序，演示通过将数学表达式字符串转化为后缀表达式，并通过后缀表达式结合栈的应用实现对算术表达式进行四则混合运算。-

Expressions computing is one of the basic issues to realize programming language, its implementation is a typical example of an application stack. Design a program that demonstrates the string by a mathematical expression into postfix notation, combined with a stack of applications to achieve by postfix arithmetic expressions four mixed computing.