英文:
the function of the head of a linked list
问题
我不知道链表头的作用是什么，它是否包含数据，还是只用来指向列表中的第一项？
有时我发现一些资源将数据存储在头部，而不仅如此，这段代码在geeks4geeks上：
/* 3. 将新节点的下一个节点设置为头部 */
new_node->next = (*head_ref);
/* 4. 将头部移动到指向新节点 */
(*head_ref) = new_node;
我不太理解它。
英文:
I don't know what is the function of the linked list header, does it contain data, or just to point to the first item in the list?
sometimes I found some resources stock data in the head, more than that this code on geeks4geeks
/* 3. Make next of new node as head */
new_node->next = (*head_ref);
/* 4. move the head to point to the new node */
(*head_ref) = new_node;
I don't understand it well
答案1
得分: 1
head_ref 是一个 Node**，也就是说 *head_ref（我在下面也称之为 head）是一个 Node*。Node* 是指向节点的指针，而 *head_ref 具体是指向列表的第一个节点的指针。
在创建一个新节点之后，执行的代码如下：
Node **head_ref Node *head Node anon1
@ 0x1000 @ 0x3000
+-----------+ +-----------+ +-----------------+
| 0x1000 ---------->| 0x3000 ------------------------------------>| next: ... |
+-----------+ +-----------+ | data: ... |
+-----------------+
Node *new_node Node anon2
@ 0x4000
+-----------+ +-----------------+
| 0x4000 ---------->| next: NULL |
+-----------+ | data: ... |
+-----------------+
在 new_node->next = (*head_ref); 之后：
Node **head_ref Node *head Node anon1
@ 0x1000 @ 0x3000
+-----------+ +-----------+ +-----------------+
| 0x1000 ---------->| 0x3000 ---------------------------------+-->| next: ... |
+-----------+ +-----------+ | | data: ... |
| +-----------------+
|
Node *new_node Node anon2 |
@ 0x4000 |
+-----------+ +-----------------+ |
| 0x4000 ---------->| next: 0x3000 -------+
+-----------+ | data: ... |
+-----------------+
在 (*head_ref) = new_node; 之后：
Node **head_ref Node *head Node anon1
@ 0x1000 @ 0x3000
+-----------+ +-----------+ +-----------------+
| 0x1000 ---------->| 0x4000 -------+ +-->| next: ... |
+-----------+ +-----------+ | | | data: ... |
| | +-----------------+
| |
Node *new_node | Node anon2 |
| @ 0x4000 |
+-----------+ | +-----------------+ |
| 0x4000 -------+-->| next: 0x3000 -------+
+-----------+ | data: ... |
+-----------------+
但如果列表为空，那么 *head_ref 最初包含 NULL，并且你会得到：
Node **head_ref Node *head
@ 0x1000
+-----------+ +-----------+
| 0x1000 ---------->| 0x4000 -------+
+-----------+ +-----------+ |
|
|
Node *new_node | Node anon2
| @ 0x4000
+-----------+ | +-----------------+
| 0x4000 -------+-->| next: NULL |
+-----------+ | data: ... |
+-----------------+
完美！（所有地址完全是虚构的，当然。）
英文:
head_ref is a Node**, which is to say that *head_ref (which I also called head below) is a Node*. A Node* is a pointer to a node, and *head_ref is specifically a pointer to the first node of the list.
Node **head_ref Node *head Node anon1 Node anon0
@ 0x1000 @ 0x3000 @ 0x2000
+-----------+ +-----------+ +-----------------+ +-----------------+
| 0x1000 ---------->| 0x3000 ---------->| next: 0x2000 ---------->| next: NULL |
+-----------+ +-----------+ | data: ... | | data: ... |
+-----------------+ +-----------------+
The code you have there is executed after creating a new node.
Node **head_ref Node *head Node anon1
@ 0x1000 @ 0x3000
+-----------+ +-----------+ +-----------------+
| 0x1000 ---------->| 0x3000 ------------------------------------>| next: ... |
+-----------+ +-----------+ | data: ... |
+-----------------+
Node *new_node Node anon2
@ 0x4000
+-----------+ +-----------------+
| 0x4000 ---------->| next: NULL |
+-----------+ | data: ... |
+-----------------+
After new_node->next = (*head_ref);:
Node **head_ref Node *head Node anon1
@ 0x1000 @ 0x3000
+-----------+ +-----------+ +-----------------+
| 0x1000 ---------->| 0x3000 ---------------------------------+-->| next: ... |
+-----------+ +-----------+ | | data: ... |
| +-----------------+
|
Node *new_node Node anon2 |
@ 0x4000 |
+-----------+ +-----------------+ |
| 0x4000 ---------->| next: 0x3000 -------+
+-----------+ | data: ... |
+-----------------+
After (*head_ref) = new_node;
Node **head_ref Node *head Node anon1
@ 0x1000 @ 0x3000
+-----------+ +-----------+ +-----------------+
| 0x1000 ---------->| 0x4000 -------+ +-->| next: ... |
+-----------+ +-----------+ | | | data: ... |
| | +-----------------+
| |
Node *new_node | Node anon2 |
| @ 0x4000 |
+-----------+ | +-----------------+ |
| 0x4000 -------+-->| next: 0x3000 -------+
+-----------+ | data: ... |
+-----------------+
But what if the list is empty? Then *head_ref starts off containing NULL, and you end up with
Node **head_ref Node *head
@ 0x1000
+-----------+ +-----------+
| 0x1000 ---------->| 0x4000 -------+
+-----------+ +-----------+ |
|
|
Node *new_node | Node anon2
| @ 0x4000
+-----------+ | +-----------------+
| 0x4000 -------+-->| next: NULL |
+-----------+ | data: ... |
+-----------------+
Perfect!
(All addresses are completely fictional, of course.)
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