说明：  设计实现一个简单的计算器，可以接受中缀数值表达式，并进行求值。 至少应该包括加、减、乘、除4种运算和括号处理； （2） 运算量可以自己约束（整数、实数等）； （3）有能力的同学可以仿照WINDOWS计算器界面
(The design and implementation of a simple calculator, can accept infix numerical expression, and evaluation. At least 4 additional operations, including adding, subtraction, multiplication, and division, should be included; (2) the amount of computation can be bound by itself (integer, real number, etc.); (3) able students can copy the WINDOWS calculator interface)