表达式类型的实现

1. 算术表达式的合法输入数据包括变量（A~Z,a~z）、常量（0－9）和二元运算符（＋，－，＊，／，^（乘幂））。程序对数据输入格式不作要求，用户需自己保证语法正确，详见用户手册说明。

2. 演示程序以人机对话的方式执行，即在计算机上显示提示信息后，由用户在键盘上输入对应的数据或命令，程序将执行相应的操作并显示下一步信息。

3. 程序执行的命令包括：

1) 根据用户输入的前缀表达式自动添加括号并建立其自然书写形式的表达式；

2）提示用户给各变量赋值；

3）建立二叉树，并基于二叉树进行求值运算，显示结果。

-expression of the realization of a type. Arithmetic expressions of the legitimate input data includes variables (A ~ Z, a-z), constants (0-9) and two yuan Operators (+,-,*,/,^( 207.142.131.235)). Procedures for data input format is not asking for, users need to ensure their correct grammar, as detailed user manual. 2. Demonstration Program in man-machine dialogue on the implementation of the computer that is displayed on the prompt message, by the user at the keyboard corresponding to the input data or orders, procedures corresponding to the implementation of the operation and display the next message. 3. Procedures for the implementation of the order include : 1) According to user input automatically adds the prefix expression brackets and the establishment of its natural forms of written