说明：  这是一个简单的计算器，当输入一个完整的计算是之后进行计算,转换成后最表达式之后再进行计算得出结果。利用堆栈实现的。
(This is a simple calculator, when the importation of a complete calculation is carried out after the calculation, converted into the most regular expressions and then calculated the results. Use stack achieved.)