在循环单链表中，链表的最后一个节点包含指向链表的第一个节点的指针。可以有循环单向链表以及循环双链表。
遍历一个循环单链表，直到到达开始的同一个节点。循环单链表类似于链表，但它没有开始也没有结束。任何节点的下一部分都不存在NULL值。
下图显示了一个循环单链表。
循环链表主要用于操作系统中的任务维护。有许多例子，循环链表用于计算机科学，包括浏览器，记录用户过去访问过的页面记录也可以以循环链表的形式保存，并且可以在点击前一个按钮时再次访问。
1. 循环链表的内存表示
在下图中，包含4个科目中学生分数的循环链表的内存表示形式。但是，该图显示了循环列表如何存储在内存中。链表的开头或head指向索引为1的元素，data部分包含分数值为13，next部分包含地址为4。它与存储在链表的第4个索引处的节点链接。
但是，由于内存中是循环链表，因此链表的最后一个节点包含链表的第一个节点的地址。
还可以在内存中有多个链表，并且不同的起始指针指向链表中的不同起始节点。 最后一个节点由其next部分标识，该部分包含链表的起始节点的地址。必须能够识别任何链表的最后一个节点，以便可以找到遍历链表时需要执行的迭代次数。
2. 循环单向链表上的操作
2.1. 插入
| 编号 | 操作 | 描述 |
|---|---|---|
| 1 | 插入开头 | 将节点添加到循环单链表的开头。 |
| 2 | 插入未尾 | 将节点添加到循环单链表的未尾。 |
2.2. 删除和遍历
| 编号 | 操作 | 描述 |
|---|---|---|
| 1 | 删除开头节点 | 删除循环单链表中的开头节点。 |
| 2 | 删除未尾节点 | 删除循环单链表中的末尾的节点。 |
| 3 | 搜索 | 将循环单链表中节点的数据项与给定数据进行比较，如果找到则返回链表中数据项所在的位置，否则返回null。 |
| 4 | 遍历 | 访问链表的每个元素至少一次，以执行某些特定操作。 |
C语言中程序循环单链表实现所有操作，参考以下示例代码 -
#include<stdio.h>
#include<stdlib.h>
struct node
{
int data;
struct node *next;
};
struct node *head;
void beginsert();
void lastinsert();
void randominsert();
void begin_delete();
void last_delete();
void random_delete();
void display();
void search();
void main()
{
int choice = 0;
while (choice != 7)
{
printf("*********Main Menu*********\n");
printf("Choose one option from the following list ...\n");
printf("===============================================\n");
printf("1.Insert in begining\n2.Insert at last\n");
printf("3.Delete from Beginning\n4.Delete from last\n");
printf("5.Search for an element\n6.Show\n7.Exit\n");
printf("Enter your choice?\n");
scanf("%d", &choice);
switch (choice)
{
case 1:
beginsert();
break;
case 2:
lastinsert();
break;
case 3:
begin_delete();
break;
case 4:
last_delete();
break;
case 5:
search();
break;
case 6:
display();
break;
case 7:
exit(0);
break;
default:
printf("Please enter valid choice..");
}
}
}
void beginsert()
{
struct node *ptr, *temp;
int item;
ptr = (struct node *)malloc(sizeof(struct node));
if (ptr == NULL)
{
printf("OVERFLOW");
}
else
{
printf("Enter the node data?");
scanf("%d", &item);
ptr->data = item;
if (head == NULL)
{
head = ptr;
ptr->next = head;
}
else
{
temp = head;
while (temp->next != head)
temp = temp->next;
ptr->next = head;
temp->next = ptr;
head = ptr;
}
printf("node inserted\n");
}
}
void lastinsert()
{
struct node *ptr, *temp;
int item;
ptr = (struct node *)malloc(sizeof(struct node));
if (ptr == NULL)
{
printf("OVERFLOW\n");
}
else
{
printf("Enter Data?");
scanf("%d", &item);
ptr->data = item;
if (head == NULL)
{
head = ptr;
ptr->next = head;
}
else
{
temp = head;
while (temp->next != head)
{
temp = temp->next;
}
temp->next = ptr;
ptr->next = head;
}
printf("node inserted\n");
}
}
void begin_delete()
{
struct node *ptr;
if (head == NULL)
{
printf("UNDERFLOW");
}
else if (head->next == head)
{
head = NULL;
free(head);
printf("node deleted\n");
}
else
{
ptr = head;
while (ptr->next != head)
ptr = ptr->next;
ptr->next = head->next;
free(head);
head = ptr->next;
printf("node deleted\n");
}
}
void last_delete()
{
struct node *ptr, *preptr;
if (head == NULL)
{
printf("UNDERFLOW");
}
else if (head->next == head)
{
head = NULL;
free(head);
printf("node deleted\n");
}
else
{
ptr = head;
while (ptr->next != head)
{
preptr = ptr;
ptr = ptr->next;
}
preptr->next = ptr->next;
free(ptr);
printf("node deleted\n");
}
}
void search()
{
struct node *ptr;
int item, i = 0, flag = 1;
ptr = head;
if (ptr == NULL)
{
printf("Empty List\n");
}
else
{
printf("Enter item which you want to search?\n");
scanf("%d", &item);
if (head->data == item)
{
printf("item found at location %d", i + 1);
flag = 0;
}
else
{
while (ptr->next != head)
{
if (ptr->data == item)
{
printf("item found at location %d ", i + 1);
flag = 0;
break;
}
else
{
flag = 1;
}
i++;
ptr = ptr->next;
}
}
if (flag != 0)
{
printf("Item not found\n");
}
}
}
void display()
{
struct node *ptr;
ptr = head;
if (head == NULL)
{
printf("nothing to print");
}
else
{
printf("printing values ... \n");
while (ptr->next != head)
{
printf("%d\n", ptr->data);
ptr = ptr->next;
}
printf("%d\n", ptr->data);
}
}
执行上面示例代码，得到以下结果 -
*********Main Menu*********
Choose one option from the following list ...
===============================================
1.Insert in begining
2.Insert at last
3.Delete from Beginning
4.Delete from last
5.Search for an element
6.Show
7.Exit
Enter your choice?
1
Enter the node data?10
node inserted
*********Main Menu*********
Choose one option from the following list ...
===============================================
1.Insert in begining
2.Insert at last
3.Delete from Beginning
4.Delete from last
5.Search for an element
6.Show
7.Exit
Enter your choice?
2
Enter Data?20
node inserted
*********Main Menu*********
Choose one option from the following list ...
===============================================
1.Insert in begining
2.Insert at last
3.Delete from Beginning
4.Delete from last
5.Search for an element
6.Show
7.Exit
Enter your choice?
2
Enter Data?30
node inserted
*********Main Menu*********
Choose one option from the following list ...
===============================================
1.Insert in begining
2.Insert at last
3.Delete from Beginning
4.Delete from last
5.Search for an element
6.Show
7.Exit
Enter your choice?
3
node deleted
*********Main Menu*********
Choose one option from the following list ...
===============================================
1.Insert in begining
2.Insert at last
3.Delete from Beginning
4.Delete from last
5.Search for an element
6.Show
7.Exit
Enter your choice?
4
node deleted
*********Main Menu*********
Choose one option from the following list ...
===============================================
1.Insert in begining
2.Insert at last
3.Delete from Beginning
4.Delete from last
5.Search for an element
6.Show
7.Exit
Enter your choice?
5
Enter item which you want to search?
20
item found at location 1
*********Main Menu*********
Choose one option from the following list ...
===============================================
1.Insert in begining
2.Insert at last
3.Delete from Beginning
4.Delete from last
5.Search for an element
6.Show
7.Exit
Enter your choice?
6
printing values ...
20
*********Main Menu*********
Choose one option from the following list ...
===============================================
1.Insert in begining
2.Insert at last
3.Delete from Beginning
4.Delete from last
5.Search for an element
6.Show
7.Exit
Enter your choice?
7