说明：  输入一个含有运算符和运算函数的表达式，可以计算出其结果，过程是利用栈根据各运算符的优先度进行数值计算，并显示结果，可循环计算
(Enter an operator and the computing function contains an expression, you can calculate a result, the process is the use of the stack the priority of the various operators to conduct the numerical calculation and display the results, the calculation can be recycled)