Because what happens in a singly-linked list -- this is trying to remove an existing entry from a list -- and there's a difference between if it's the first entry or whether it's an entry in the middle.
因为在单向链表中—— 这里是试图将一个已存在的入口移出链表—— 不同之处在于这是第一个入口 还是一个位于中间的入口。