postfixexpression

代码说明：

  运用栈的基本运算。（进栈、出栈） 依次扫描表达式的操作数并入栈，遇到运算符时，将后进的两个操作数出栈，对其进行运算，将运算结果入栈。照此方法，进行下去，一直到执行完最后一个运算符，输出结果。(The use of basic computing stack. (Into the stack, out of the stack) followed by scanning the expression operands and stack encountered operator, it will be backward out of the two operand stack and to carry out operations, the result would be computing stack. Do the same method, go on, until the implementation of the End of the last operator, output.)