语法分析正确后就可以求值了，求值时用到一个操作数堆栈和操作符堆栈，以及一个算符优先表（存储了运算符之间的优先关系），具体细节可以阅读源码。-syntax analysis is correct after it evaluates the value used for a number of operations and the operator stacks stack, and an operator priority list (stored Operators relations bet