四则混合运算表达式的处理。对输入的逆波兰表达式和正常顺序（带括号、运算符优先级）的表达式构造二叉表达式树、输出前序中序后序遍历结果、树状输出、表达式赋值及运算-Handle mixed operation expression of the four. For Reverse Polish Notation input and the normal order (with brackets, operator precedence) expression construct binary expression tree traversal sequence after sequence output results in the preamble, the tree output, expression　uation and operation