双向循环线性链表，链表初始化、插入结点、修改结点的值、删除特定结点、打印链表、销毁链表。在以上的基础下实现一元多项式的相加。-Two-cycle linear list, linked list is initialized, insert node, modify the value of node to delete a specific node, print list, the destruction list. Basis in more than a dollar to achiev