十进制四则运算计算器，用户输入一串四则运算表达式，系统将其以二叉链表形式存储，并输出先序、中序和后序遍历的结点顺序，最后利用栈计算后缀表达式的值，输出运算结果。-Decimal four arithmetic calculator, the user enters a string of four arithmetic expressions, the system to be stored in the binary tree, and outputs the first sequence,