循环双向链表是一种更复杂的数据结构类型，它的节点包含指向其前一节点以及下一节点的指针。 循环双向链表在任何节点中都不包含NULL。 链表的最后一个节点包含列表的第一个节点的地址。 链表的第一个节点还包含的前一个指针是指向最后一个节点的地址。
循环双向链表如下图所示 -
image
由于循环双向链表在其结构中包含三个部分，因此每个节点需要更多空间和更昂贵的基本操作。 但是，循环双向链表提供了对指针更方便的操作，搜索效率提高了两倍。
1. 循环双向链表的内存管理
下图显示了为循环双向链表分配内存的方式。 变量head包含链表的第一个元素的地址，即地址1，因此链表的起始节点包含数据A存储在地址1中。因此，链表的每个节点应该具有三个部分，起始节点包含最后一个(也是前一个)节点的地址，即8和下一个节点，即4。链表的最后一个节点，存储在地址8并包含数据6，包含链表的第一个节点的地址，如图中所示，即地址1。在循环双向链表中，最后一个节点由第一个节点的地址标识，该节点存储在最后一个节点的next中，因此包含第一个节点地址的节点实际上是该节点的最后一个节点。
image
2. 循环双向链表的操作
可以在循环双链表上执行各种操作。循环双链表的节点结构类似于双向链表。 但是，循环双向链表上的操作如下表所述。
| 编号 | 操作 | 描述 |
|---|---|---|
| 1 | 在开头插入节点 | 在循环双向链表的开头添加节点。 |
| 2 | 在末尾插入节点 | 在循环双向链表的末尾添加节点。 |
| 3 | 删除开头节点 | 删除循环双向链表开头的节点。 |
| 4 | 删除末尾节点 | 删除循环双向链表末尾的节点。 |
在循环双向链表中遍历和搜索类似于循环单链表中的遍历和搜索，因此不再说明。
C语言实现的示例代码 -
#include<stdio.h>
#include<stdlib.h>
struct node
{
struct node *prev;
struct node *next;
int data;
};
struct node *head;
void insertion_beginning();
void insertion_last();
void deletion_beginning();
void deletion_last();
void display();
void search();
void main()
{
int choice = 0;
while (choice != 9)
{
printf("*********Main Menu*********\n");
printf("Choose one option from the following list ...\n");
printf("===============================================\n");
printf("1.Insert in Beginning\n2.Insert at last\n");
printf("3.Delete from Beginning\n4.Delete from last\n");
prinft("5.Search\n6.Show\n7.Exit\n");
printf("Enter your choice?\n");
scanf("\n%d", &choice);
switch (choice)
{
case 1:
insertion_beginning();
break;
case 2:
insertion_last();
break;
case 3:
deletion_beginning();
break;
case 4:
deletion_last();
break;
case 5:
search();
break;
case 6:
display();
break;
case 7:
exit(0);
break;
default:
printf("Please enter valid choice..");
}
}
}
void insertion_beginning()
{
struct node *ptr, *temp;
int item;
ptr = (struct node *)malloc(sizeof(struct node));
if (ptr == NULL)
{
printf("OVERFLOW");
}
else
{
printf("Enter Item value");
scanf("%d", &item);
ptr->data = item;
if (head == NULL)
{
head = ptr;
ptr->next = head;
ptr->prev = head;
}
else
{
temp = head;
while (temp->next != head)
{
temp = temp->next;
}
temp->next = ptr;
ptr->prev = temp;
head->prev = ptr;
ptr->next = head;
head = ptr;
}
printf("Node inserted\n");
}
}
void insertion_last()
{
struct node *ptr, *temp;
int item;
ptr = (struct node *) malloc(sizeof(struct node));
if (ptr == NULL)
{
printf("OVERFLOW");
}
else
{
printf("Enter value");
scanf("%d", &item);
ptr->data = item;
if (head == NULL)
{
head = ptr;
ptr->next = head;
ptr->prev = head;
}
else
{
temp = head;
while (temp->next != head)
{
temp = temp->next;
}
temp->next = ptr;
ptr->prev = temp;
head->prev = ptr;
ptr->next = head;
}
}
printf("node inserted\n");
}
void deletion_beginning()
{
struct node *temp;
if (head == NULL)
{
printf("UNDERFLOW");
}
else if (head->next == head)
{
head = NULL;
free(head);
printf("node deleted\n");
}
else
{
temp = head;
while (temp->next != head)
{
temp = temp->next;
}
temp->next = head->next;
head->next->prev = temp;
free(head);
head = temp->next;
}
}
void deletion_last()
{
struct node *ptr;
if (head == NULL)
{
printf("UNDERFLOW");
}
else if (head->next == head)
{
head = NULL;
free(head);
printf("node deleted\n");
}
else
{
ptr = head;
if (ptr->next != head)
{
ptr = ptr->next;
}
ptr->prev->next = head;
head->prev = ptr->prev;
free(ptr);
printf("node deleted\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);
}
}
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");
}
}
}
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 Beginning
2.Insert at last
3.Delete from Beginning
4.Delete from last
5.Search
6.Show
7.Exit
Enter your choice?
1
Enter Item value123
Node inserted
*********Main Menu*********
Choose one option from the following list ...
===============================================
1.Insert in Beginning
2.Insert at last
3.Delete from Beginning
4.Delete from last
5.Search
6.Show
7.Exit
Enter your choice?
2
Enter value234
node inserted
*********Main Menu*********
Choose one option from the following list ...
===============================================
1.Insert in Beginning
2.Insert at last
3.Delete from Beginning
4.Delete from last
5.Search
6.Show
7.Exit
Enter your choice?
1
Enter Item value90
Node inserted
*********Main Menu*********
Choose one option from the following list ...
===============================================
1.Insert in Beginning
2.Insert at last
3.Delete from Beginning
4.Delete from last
5.Search
6.Show
7.Exit
Enter your choice?
2
Enter value80
node inserted
*********Main Menu*********
Choose one option from the following list ...
===============================================
1.Insert in Beginning
2.Insert at last
3.Delete from Beginning
4.Delete from last
5.Search
6.Show
7.Exit
Enter your choice?
3
*********Main Menu*********
Choose one option from the following list ...
===============================================
1.Insert in Beginning
2.Insert at last
3.Delete from Beginning
4.Delete from last
5.Search
6.Show
7.Exit
Enter your choice?
4
node deleted
*********Main Menu*********
Choose one option from the following list ...
===============================================
1.Insert in Beginning
2.Insert at last
3.Delete from Beginning
4.Delete from last
5.Search
6.Show
7.Exit
Enter your choice?
6
printing values ...
123
*********Main Menu*********
Choose one option from the following list ...
===============================================
1.Insert in Beginning
2.Insert at last
3.Delete from Beginning
4.Delete from last
5.Search
6.Show
7.Exit
Enter your choice?
5
Enter item which you want to search?
123
item found at location 1
*********Main Menu*********
Choose one option from the following list ...
============================================
1.Insert in Beginning
2.Insert at last
3.Delete from Beginning
4.Delete from last
5.Search
6.Show
7.Exit
Enter your choice?
7
