编写一个程序，通过前缀表达式来构造一个与之对应的算术表达式并存入二叉树中，通过中序遍历二叉树输出表达式及对其进行变量赋值，通过后序遍历二叉树计算表达式的值。(A program is written by prefix expression to construct an arithmetic expression corresponding to it and store it into two binary tree. Through traversing two fork tree output expression and variable assignment, it calculates the expression value through traversal of two fork tree.)