双向循环链表中要在结点 p 之前插入新结点 s（均非空），以下指针操作正确的是（ ）。
- A. 1 s -> next = p; 2 p -> prev = s; 3 p -> next = s; 4 s -> prev = p;
- B. 1 s -> prev = p; 2 s -> next = p -> next; 3 p -> next -> prev = s; 4 p -> next = s;
- C. 1 s -> next = p; 2 s -> prev = p->prev; 3 p -> prev -> next = s; 4 p -> prev = s;
- D. 1 s -> next = p; 2 s -> prev = nullptr; 3 p -> prev = s;
正确答案：C