对算术表达式进行词法和语法分析，思路:大体遵循着编译原理中提到的词法和语法分析步骤,即先对表达式进行区分为操作符或标志符,将其存放在符号表中，而后从符号表中识别符号类别和操作符优先级存放在二叉树中。(Analysis of lexical and syntax of arithmetic expressions: roughly follow the principle of compiling ideas mentioned in the lexical and syntax analysis