已知有一个单向循环链表，其每一个结点中含三个域：pre，data和next，其中data为数据域，next为指向后继结点的指针域，pre也为指针域，但它的值为空（NULL），试编写算法将此单向循环链表改为双向循环链表，即使pre成为指向前驱结点的指针域。-Known to have a one-way circulation list, which each node contains three domains: pre, data and next, in which data for the data field, next to point to the successor node pointer domain, pre-domain for the pointer, but its value is empty (NULL), try to prepare this one-way circulation list algorithm changed to a two-way circulation list, even if the pre-node as a precursor to the target domain.