使用单链表实现多项式的加法。在进行添加新项（添加新节点）时自动排序，若项数为0，自动删除该项。在进行加法运算时自动排序，若项数为0，自动删除该项。-Single linked list to achieve polynomial addition. During the Add New Item (add new nodes) to automatically sort, if the number of entries to 0, automatically delete the item. Automatically sorted during the addition operation, if the number of entries to 0, automatically delete the item.