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 procedures, namely the expressions are divided into an operator or symbol, it will be stored in the symbol table, and then from the symbol table identification symbol categories and operator precedence stored in the two binary tree.)