设计一个四则表达式分析软件，其接受从文件或键盘输入的四则表达式（运算对象支持整数和变量），给出该表达式的二叉树表达形式，并在键盘上按后续遍历方式输出该二叉树。

例如，输入a+b*c，输出 a b c * +

提示：参考下列的设计指导:

递归下降分析法实验设计思想及算法

为G的每个非终结符号U构造一个递归过程，不妨命名为U。U的产生式的右边指出这个过程的代码结构：

(1)若是终结符号，则和向前看符号对照，若匹配则向前进一个符号；否则出错。

(2)若是非终结符号，则调用与此非终结符对应的过程。当A的右部有多个产生式时,可用选择结构实现。-Design a four-expression analysis software, which accepts four expressions from a file or keyboard input (support integer operands and variables), given expression in the form of a binary tree of the expression, and on the keyboard, press the subsequent traversal of the binary output.

     　For example, enter a+ b* c, output a b c*+

     　Note: Refer to the following design guidelines:

       　Recursive descent experimental design ideas and algorithm analysis

         　To construct each nonterminal U G a recursive process, it may be named as U. U generates right-hand side indicates the code structure of the process:

         　(1) If the end of the symbol, and lookaheads contrast, if the match is to advance a symbol　otherwise error.

         　(2) If the non-terminal symbol, then call this nonterminal corresponding process. When there are multiple right part A of production, s