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（ ）。
p
s
s -> next = p; p -> prev = s; p -> next = s; s -> prev = p;
s -> prev = p; s -> next = p -> next; p -> next -> prev = s; p -> next = s;
s -> next = p; s -> prev = p->prev; p -> prev -> next = s; p -> prev = s;
s -> next = p; s -> prev = nullptr; p -> prev = s;