向一个栈顶为 hs 的链式栈中插入一个指针为 s 的结点时，应执行（ ）。
hs
s
hs->next = s;
s->next = hs; hs = s;
s->next = hs->next; hs->next = s;
s->next = hs; hs = hs->next;