C++之父的《程序设计与实践》书中的简易计算器改进版本，通过文法实现“（）”，“*”，“/”等优先级运算，通过改进实现花括号“{}”的运算-C++ the father of the " program design and practice" simple calculator improved version of the book, through the grammar to achieve " ()" , " *" , " /&