考虑用双向链表来实现一个有序表，使得能在这个表中进行正向和反向搜索。若指针p 总是指向最后成功搜索到的结点，搜索可以从p 指示的结点出发沿任一方向进行。试根据这种情况编写一个函数search(head, p, key)，检索具有关键码key 的结点，并相应地修改p。-Consider the use of two-way linked list to implement a sorted list in this table allows both forward and reverse search. If the pointer p always points to the last successful search nodes, the search direction from the node p starting in either direction. According to this test case to write a function search (head, p, key), to retrieve the key code with key nodes, and modify p.