Node *head 和 Node **head 有什么区别? [英] What is the difference between Node *head versus Node **head?
问题描述
我正在编写一个 C 代码来查找链表的中间位置.我理解逻辑,但无法弄清楚如何使用指针.Node *head
和 Node** head_ref
的工作方式有何不同?
I am writing a C code to find the middle of linked list. I understood the logic but was unable to figure out how pointers are being used. What is the difference in the way Node *head
and Node** head_ref
work?
void middle(struct Node *head) ;
void push(struct Node** head_ref, int new_data) ;
推荐答案
在第一个函数头中,*head
是一个指向分配在内存中某处的节点对象的指针:
In the first function header, *head
is a pointer to a node object which is allocated somewhere in memory:
void middle(struct Node *head);
_____________________
| |
*head --> | Node object |
| [val=1][*next=NULL] |
|_____________________|
而在第二个函数头中,**head_ref
是指向内存中某处节点对象的指针:
while in the second function header, **head_ref
is a pointer to a pointer to a node object somewhere in memory:
void push(struct Node** head_ref, int new_data);
_____________________
| |
*head --> | Node object |
^ | [val=1][*next=NULL] |
| |_____________________|
|
**head_ref
这是另一个间接层.为什么需要第二个构造?好吧,如果我想修改在我的函数范围之外分配的东西,我需要一个指向它的内存位置的指针.在第一个示例中,我可以取消引用 *head
指针(使用 head->
)来访问内存中的底层 Node
对象并生成对其进行修改或访问其属性.
This is another layer of indirection. Why is the second construct necessary? Well, if I want to modify something allocated outside of my function scope, I need a pointer to its memory location. In the first example, I can dereference the *head
pointer (with head->
) to access the underlying Node
object in memory and make modifications to it or access its properties.
把这个逻辑带到下一个间接层次,如果我想把一个新的Node
对象推到列表的前面,使它成为新的头部,我需要修改head
指针本身.就像我想用一个指针操作 Node
对象一样,现在我需要一个指向 *head
的指针(它恰好是一个指针而不是一个 Node
对象)以对其进行修改.
Taking this logic to the next level of indirection, if I want to push a new Node
object onto the front of the list to make it the new head, I need to modify the head
pointer itself. Just as when I wanted to manipulate the Node
object with a pointer, now I need a pointer to *head
(which simply happens to be a pointer rather than a Node
object) in order to modify it.
这是push
的内容和一个before/after函数调用:
Here's the contents of push
and a before/after function call:
void push(struct Node** head_ref, int new_data) {
Node *new_head = malloc(sizeof(Node)); // allocate memory for the new head node
new_head->data = new_data; // set its value
new_head->next = *head_ref; // make it point to the
// old head pointer
*head_ref = new_head; // make it the new head by modifying
// the old head pointer directly
}
/* Before push */
_____________________
| |
*head --> | Node object |
^ | [val=1][*next=NULL] |
| |_____________________|
|
**head_ref
/* After calling push(&head, 2);
* ^
* `&` operator gets the memory address of `head`,
* which is a pointer.
*/
_________________ _____________________
| | | |
*head --> | Node object | | Node object |
^ | [val=2] [*next]----->| [val=1][*next=NULL] |
| |_________________| |_____________________|
|
**head_ref
这是一个完整的例子:
#include <stdio.h>
#include <stdlib.h>
typedef struct Node {
int data;
struct Node *next;
} Node;
void push(Node** head_ref, int new_data) {
Node *new_head = malloc(sizeof(Node));
new_head->data = new_data;
new_head->next = *head_ref;
*head_ref = new_head;
}
void print(Node *head) {
while (head) {
printf("%d->", head->data);
head = head->next;
}
puts("NULL");
}
void free_list(Node *head) {
while (head) {
Node *tmp = head;
head = head->next;
free(tmp);
}
}
int main() {
Node *head = malloc(sizeof(Node));
head->next = NULL;
head->data = 1;
printf("Before push:
");
print(head);
push(&head, 2);
printf("
After push:
");
print(head);
free_list(head);
return 0;
}
输出:
Before push:
1->NULL
After push:
2->1->NULL
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