说明：   逆波兰表达式求值问题 　　　 1.基于堆栈的基本操作。 2.若表达式正确，界面自动输出后缀表达式的值，否则输出表达式错误的信息,将结果置为0。 3.操作数和操作符均由键盘键入。
(Reverse Polish expression evaluation questions 1. Based on the basic operation of the stack. (2) If the expression is correct, the interface is automatically output postfix expression value, otherwise error message output expression, the result is set to 0. 3 operands and operators by the keyboard.)