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上一节课，我们学了线性表 单向存储结构（也就是单链表），这个是企业常用的技术，且是后面各种的基本，一定要牢牢掌握，如果没有掌握，下面的课程会云里雾里。

一 ，循环链表

1、什么是循环链表

— 概念上：
1、任何数据元素都有一个前驱和一个后继
2、所有的数据元素的关系构成一个逻辑的环

— 实现上：
1、循环链表是一种特殊的单链表
2、尾结点的指针域保存了首结点的地址

2、循环链表的逻辑构成

二 循环链表的插入示意图

头插法第一次插入

头插法非第一次插入

删除非第一个元素

删除第一个元素

三 代码实现

.h实现

#ifndef _003CIRCLELIST_H_
#define _003CIRCLELIST_H_typedef void CircleList;  //要返回给上层的list 的首地址为 void *,为了阅读起名为CircleListtypedef struct _tag_CircleListNode  //list 中的节点
{struct _tag_CircleListNode* next;
}CircleListNode;//创建循环链表
CircleList* CircleList_Create();//销毁循环链表
void CircleList_Destroy(CircleList* list);//清空循环列表
void CircleList_Clear(CircleList* list);//循环列表中目前存在的元素个数
int CircleList_Length(CircleList* list);//给循环列表中插入元素，node为要插入的元素的值，pos为位置
int CircleList_Insert(CircleList* list, CircleListNode* node, int pos);//从循环列表中的pos 位置获得具体的值
CircleListNode* CircleList_Get(CircleList* list, int pos);//从循环列表中删除pos位置的数据
CircleListNode* CircleList_Delete(CircleList* list, int pos);//从循环列表中删除 数据 为node 的点
CircleListNode* CircleList_DeleteNode(CircleList* list, CircleListNode* node);CircleListNode* CircleList_Reset(CircleList* list);CircleListNode* CircleList_Current(CircleList* list);CircleListNode* CircleList_Next(CircleList* list);#endif

底层实现

#include "003CircleList.h"
#include "stdio.h"
#include "stdlib.h"
#include <string.h>typedef struct _tag_CircleList{CircleListNode header;CircleListNode* slider; //多了一个游标int length;
} TCircleList;CircleList* CircleList_Create(){TCircleList* ret = (TCircleList*)malloc(sizeof(TCircleList));if (ret == NULL) {printf("CircleList_Create func malloc error\n");return ret;}memset(ret,0,sizeof(TCircleList));ret->length = 0;ret->header.next = NULL;ret->slider = NULL;return ret;
}void CircleList_Destroy(CircleList* list) // O(1)
{free(list);
}void CircleList_Clear(CircleList* list){TCircleList* sList = (TCircleList*)list;if (sList != NULL){sList->length = 0;sList->header.next = NULL;sList->slider = NULL;}
}int CircleList_Length(CircleList* list){TCircleList* sList = (TCircleList*)list;int ret = -1;if (sList != NULL){ret = sList->length;}return ret;
}int CircleList_Insert(CircleList* list, CircleListNode* node, int pos) // O(n)
{TCircleList* sList = (TCircleList*)list;int ret = (sList != NULL) && (pos >= 0) && (node != NULL);int i = 0;if (ret){CircleListNode* current = (CircleListNode*)sList;//current指向头部for (i = 0; (i < pos) && (current->next != NULL); i++){current = current->next;}//假设我们要插入的是pos =3，头结点不算，下来从0,1,2,3,4,5,6开始计算//循环完成后，current刚好是在 pos=2的位置，//要变成的是 2  node   3 ，也就是说node->next要是3node->next = current->next;//current的->next，现在也是2，指向新的节点nodecurrent->next = node;if (sList->length == 0){//如果是第一次插入将slider的指向nodesList->slider = node;}sList->length++;//如果是头插法，还需要做事情，让最后一个元素链接到这个新节点，if (current == (CircleListNode*)sList) {CircleListNode * last = CircleList_Get(list,sList->length-1);last->next = node;}//此处要理解，需结合图来看，后续会将 头插法，尾插法，中间插入法的三种图示画一下，方便理解}return ret;
}CircleListNode* CircleList_Get(CircleList* list, int pos) // O(n)
{TCircleList* sList = (TCircleList*)list;CircleListNode* ret = NULL;int i = 0;if ((sList != NULL) && (pos >= 0)){CircleListNode* current = (CircleListNode*)sList;for (i = 0; i < pos; i++){current = current->next;}ret = current->next;}return ret;
}CircleListNode* CircleList_Delete(CircleList* list, int pos) // O(n)
{TCircleList* sList = (TCircleList*)list;CircleListNode* ret = NULL;int i = 0;if ((sList != NULL) && (pos >= 0)){CircleListNode* current = (CircleListNode*)sList;CircleListNode* first = sList->header.next;CircleListNode* last = (CircleListNode*)CircleList_Get(sList, sList->length - 1);for (i = 0; i < pos; i++){current = current->next;}ret = current->next;current->next = ret->next;sList->length--;//如果删除的第一个结点。要额外处理if (first == ret){//让头结点的next要重新指向，指向的内容是保存在 被删除的节点的next中的。sList->header.next = ret->next;//让最后一个节点的next也要重新指向，指向的内容是保存在 被删除的节点的next中的。last->next = ret->next;}//如果删除的元素刚好是 游标指向的元素，则将游标往下移动if (sList->slider == ret){sList->slider = ret->next;}//如果list只有一个元素，删除后，就没有元素了，那么就需要将if (sList->length == 0){sList->header.next = NULL;sList->slider = NULL;}}return ret;
}CircleListNode* CircleList_DeleteNode(CircleList* list, CircleListNode* node) // O(n)
{TCircleList* sList = (TCircleList*)list;CircleListNode* ret = NULL;int i = 0;if (sList != NULL){CircleListNode* current = (CircleListNode*)sList;for (i = 0; i < sList->length; i++){if (current->next == node){ret = current->next;break;}current = current->next;}if (ret != NULL){CircleList_Delete(sList, i);}}return ret;
}CircleListNode* CircleList_Reset(CircleList* list) // O(1)
{TCircleList* sList = (TCircleList*)list;CircleListNode* ret = NULL;if (sList != NULL){sList->slider = sList->header.next;ret = sList->slider;}return ret;
}CircleListNode* CircleList_Current(CircleList* list) // O(1)
{TCircleList* sList = (TCircleList*)list;CircleListNode* ret = NULL;if (sList != NULL){ret = sList->slider;}return ret;
}CircleListNode* CircleList_Next(CircleList* list) // O(1)
{TCircleList* sList = (TCircleList*)list;CircleListNode* ret = NULL;if ((sList != NULL) && (sList->slider != NULL)){ret = sList->slider;sList->slider = ret->next;}return ret;
}

测试代码

#include "iostream"
#include <stdio.h>
#include <stdlib.h>extern "C" {
#include "003CircleList.h"
}using namespace std;struct Value
{CircleListNode header;int v;
};int main(int argc, char *argv[]){int i = 0;CircleList* list = CircleList_Create();struct Value v1;struct Value v2;struct Value v3;struct Value v4;struct Value v5;struct Value v6;struct Value v7;struct Value v8;v1.v = 1;v2.v = 2;v3.v = 3;v4.v = 4;v5.v = 5;v6.v = 6;v7.v = 7;v8.v = 8;CircleList_Insert(list, (CircleListNode*)&v1, CircleList_Length(list));CircleList_Insert(list, (CircleListNode*)&v2, CircleList_Length(list));CircleList_Insert(list, (CircleListNode*)&v3, CircleList_Length(list));CircleList_Insert(list, (CircleListNode*)&v4, CircleList_Length(list));for (i = 0; i <  CircleList_Length(list); i++){struct Value* pv = (struct Value*)CircleList_Get(list, i);printf("%d\n", pv->v);}//注意这里，这时候list除了头结点外，只有4个元素，1,2,3,4，对应0,1,2,3//代码中插入的pos =5，相当于在1和2中间插入一个5.CircleList_Insert(list, (CircleListNode*)&v5, 5);//因此如下的循环后，打印出来的是 1,5,2,3,4for (i = 0; i < CircleList_Length(list); i++){struct Value* pv = (struct Value*)CircleList_Get(list, i);printf("%d\n", pv->v);}CircleList_Delete(list, 0);//删除第一个元素，将1删除了//再次打印是 5 2 3 4  5 2 3 4for (i = 0; i < 2 * CircleList_Length(list); i++){struct Value* pv = (struct Value*)CircleList_Get(list, i);printf("%d\n", pv->v);}printf("\n");while (CircleList_Length(list) > 0){struct Value* pv = (struct Value*)CircleList_Delete(list, 0);printf("%d\n", pv->v);}printf("aaaaaa\n");CircleList_Insert(list, (CircleListNode*)&v1, 0);CircleList_Insert(list, (CircleListNode*)&v2, 0);CircleList_Insert(list, (CircleListNode*)&v3, 0);CircleList_Insert(list, (CircleListNode*)&v4, 0);CircleList_Insert(list, (CircleListNode*)&v5, 0);CircleList_Insert(list, (CircleListNode*)&v6, 0);CircleList_Insert(list, (CircleListNode*)&v7, 0);CircleList_Insert(list, (CircleListNode*)&v8, 0);//注意，这里是用的头插法，因此是8,7,6,5,4,3,2,1，但是第一个插入的是1，因此游标指向1，又因为是循环的，因此下一个是8，结果是1,8,7,6,5,4,3,2for (i = 0; i < CircleList_Length(list); i++){struct Value* pv = (struct Value*)CircleList_Next(list);printf("%d\n", pv->v);}printf("bbbbbbbbbb\n");//游标reset 是指向的第一个元素CircleList_Reset(list);//1,2,3 将3剔除队列的游戏，游标reset后，指向的是8，因此123，将3剔除队列的有些结果为 6,3，8,4,7，1,5,2while (CircleList_Length(list) > 0){struct Value* pv = NULL;for (i = 1; i < 3; i++){CircleList_Next(list);}printf("ccc\n");//游标reset之后，指向数字8，往后移动了2次，就是指向6pv = (struct Value*)CircleList_Current(list);printf("%d\n", pv->v);CircleList_DeleteNode(list, (CircleListNode*)pv);}CircleList_Destroy(list);return 0;}