（1）按逆波兰表达式建立树 (2) 输出表达式树的各种遍历的结果。 (3) 打印表达式树。 (4) 删除表达式树。 -(1) The establishment of trees by Reverse Polish Notation (2) output a variety of expression tree traversal results. (3) Print the expression tree. (4) Delete an expression tree.