新建压缩文件

代码说明：

  （1）设计一个“简易计算器”，主要实现判断输入的表达式是否合法将中缀表达式转换成二叉表达式树。后序遍历求出表达式的值。能判断输入的表达式是否合法。 （2）用链式存储存储信息，求值时可以采用递归的思想，对表达式进行后序遍历。 （3）将操作结果保存到外部文件。((1) Design a "simple calculator" to judge whether the input expression is legitimate or not and convert infix expression into binary expression tree. The value of the expression is obtained by post-order traversal. Can judge whether the input expression is legitimate. (2) Store information in chain mode. When evaluating the value, we can use the idea of recursion to carry out post-order traversal of the expression. (3) To save the operation results to external files.)