前言
循环链表是另一种形式的链式存储结构。它的特点是表中最后一个结点的指针域指向头结点,整个链表形成一个环。
单循环链表
单循环链表——在单链表中,将终端结点的指针域NULL改为指向表头结点或开始结点即可。如下图(a)所示:
注意:
循环链表的操作和线性链表基本一样,差别仅在于算法中的循环条件不是p或p->next是否为空,而是它们是否等于头指针(head)。如下图(b)所示:
单链表的问题:如果不从头结点出发,就无法访问全部结点。
解决方法:只需要将单链表中的终端结点的指针由空指针改为指向头结点
单循环链表的代码实现:
single_list.c
#include <stdio.h>
#include <stdbool.h> //c99才支持bool类型
struct single_list
{
struct single_list *next;
int val;
};
struct single_list_head
{
struct single_list *head;
};
//判断循环链表是否为空
bool is_empty(struct single_list_head *head)
{
return head->head == NULL;
}
//遍历单链表
void dump(struct single_list_head *head)
{
struct single_list *tmp = head->head;
int idx = 0;
while (tmp)
{
printf("[%02d]: %08d\n", idx++, tmp->val);
tmp = tmp->next;
}
}
//插入
void insert(struct single_list **prev, struct single_list *elem)
{
if (!prev)
return;
elem->next = *prev;
*prev = elem;
}
//插入头结点
void insert_head(struct single_list_head *head, struct single_list *elem)
{
insert(&head->head, elem);
}
//删除
struct single_list* del(struct single_list **prev)
{
struct single_list *tmp;
if (!prev)
return NULL;
if (*prev == NULL)
return NULL;
tmp = *prev; // 要删除的结点
*prev = (*prev)->next; // 记录待删除结点的下一个结点
tmp->next = NULL;
return tmp;
};
//删除头节点
struct single_list* delete_head(struct single_list_head* head)
{
return del(&head->head);
};
//返回结点所在位置
struct single_list** search(struct single_list_head* head, int target)
{
struct single_list **prev, *tmp;
for (prev = &head->head, tmp = *prev;
tmp && (tmp->val < target);
prev = &tmp->next, tmp = *prev)
;
return prev;
};
//倒序
void reverse(struct single_list_head* head)
{
struct single_list_head tmp = {NULL};
struct single_list *elem;
while (!is_empty(head))
{
elem = delete_head(head);
insert_head(&tmp, elem);
}
head->head = tmp.head;
}
//判断链表是否形成一个环
bool is_cyclic(struct single_list_head* head)
{
struct single_list *s1, *s2;
s1 = s2 = head->head;
while(s1 && s2)
{
s1 = s1->next;
s2 = s2->next ? s2->next->next:s2->next;
if (s1 == s2)
return true;
}
return false;
}
//取元素中间值
struct single_list* middle(struct single_list_head* head)
{
struct single_list *s1, *s2;
struct single_list pseudo_head;
pseudo_head.next = head->head;
s1 = s2 = &pseudo_head;
while (true) {
if (!s2 || !s2->next)
return s1;
s1 = s1->next;
s2 = s2->next->next;
}
return NULL;
};
int main(int argc, char **argv)
{
struct single_list_head head = {NULL};
struct single_list lists[10];
struct single_list **prev;
int idx;
for (idx = 0; idx < 10; idx++)
{
lists[idx].val = idx;
lists[idx].next = NULL;
}
//新插入的结点作为第一个结点
insert_head(&head, &lists[6]);
insert_head(&head, &lists[5]);
insert_head(&head, &lists[4]);
insert_head(&head, &lists[1]);
insert_head(&head, &lists[0]);
printf("=== insert 0, 1, 4, 5, 6\n");
dump(&head);
prev = search(&head, 2);
insert(prev, &lists[2]);
printf("=== insert 2\n");
dump(&head);
printf("middle elem is %d\n", middle(&head)->val);
prev = search(&head, 2);
if ((*prev) && ((*prev)->val == 2))
printf("The list contains %d\n",(*prev)->val);
else
printf("The list not contains %d\n",(*prev)->val);
del(prev); //删除
prev = search(&head, 2);
printf("After remove 2\n");
if ((*prev) && ((*prev)->val == 2))
printf("The list contains %d\n",(*prev)->val);
else
printf("The list not contains %d\n",(*prev)->val);
dump(&head);
printf("After reverse \n");
reverse(&head);
dump(&head);
printf("middle elem is %d\n", middle(&head)->val);
lists[0].next = &lists[6];
printf("list is%s cyclic\n", is_cyclic(&head)?"":" not");//判断是否有环
return 0;
}
输出结果: