不创建头结点的链表。满足以下功能：列举表元素，插入、删除元素，清空表，查询元素值，查询元素位置，列举前驱元素，列举后继元素，退出程序。-Do not create a head node of the list. Meet the following functions: lists of elements, insert, delete elements, empty table, query element values, query element position, list of pr