循环双向链表的实现

循环双向链表结构如下图所示

再看下双向链表

循环双向链表在双向链表的基础上，将双向链表的最后一个节点的next指针，指向了链表的第一个节点，而第一个节点的prev指针指向了最后一个节点。

循环双向链表和双向链表的主要区别如下

循环双向链表：可以看成是一个环，从任意一个节点出发，都可以访问到整个链表；
双向链表：可以看成是一条直线，只能从头访问到结尾，或者是从结尾访问到头；

如果想了解双向链表的实现，可以看这里 双向链表的实现

下面是循环双向链表的代码实现

#include <stdio.h>#include <string.h>#include <stdlib.h>#include <stdbool.h>#define LIST_MAX_LEN 10typedef int ElemType;typedef struct Node_T{    ElemType value;    int length;    struct Node_T * prev;    struct Node_T * next;}Node;typedef struct Node_T DList;DList * doubly_linked_list_init(void){    DList * dlist = (DList *)malloc(sizeof(DList));    if(!dlist)    {        printf("malloc dlist fail!\n");        return NULL;    }    memset(dlist, 0, sizeof(DList));    dlist->length = 0;    dlist->prev = NULL;    dlist->next = NULL;     return dlist;}bool doubly_linked_list_is_empty(DList * dlist){       return dlist->length == 0;}bool double_linked_list_is_full(DList * dlist){    if(dlist->length >= LIST_MAX_LEN)        return true;    else         return false;}/*  insert value at the tail of the list   */void doubly_linked_list_insert(DList * dlist, ElemType value){    DList * pDList, * insertElem;    pDList = dlist;    if(dlist==NULL)    {        printf("list is NULL\n");        return;    }    if(true == double_linked_list_is_full(dlist))    {        printf("list is full\n");        return;    }    /* Find the tail of the list */    while(pDList->next)    {        pDList = pDList->next;    }    insertElem = (DList *)malloc(sizeof(DList));    if(!insertElem)    {        printf("malloc insertElem fail!\n");        return;    }    memset(insertElem, 0, sizeof(DList));    insertElem->next = NULL;    insertElem->value = value;    pDList->next = insertElem;    insertElem->prev = pDList;    dlist->length++;}/*  delete value by the param index   */void doubly_linked_list_delete_by_index(DList * dlist, int index){    int count = 0;    DList * pDList = dlist;    if(dlist==NULL || dlist->next==NULL)    {        printf("list is NULL\n");        return;    }    if(index<=0 || index>=LIST_MAX_LEN || index>dlist->length)    {        printf("insert index is error\n");        return;    }    while(pDList->next && count<index)    {        pDList = pDList->next;        count ++;    }    if(pDList->next == NULL)    {        pDList->prev->next = pDList->next;    }    else    {        pDList->next->prev = pDList->prev;        pDList->prev->next = pDList->next;    }    dlist->length--;    free(pDList);       /* It need to free the delete element */}void doubly_linked_list_traverse_fordward(DList * dlist){       printf("fordward traverse : ");    while(dlist->next)    {        printf("%3d  ", dlist->next->value);    // There have the head fo the Doubly Linked List        dlist = dlist->next;    }    printf("\n");}void doubly_linked_list_traverse_backward(DList * dlist){    printf("backward traverse : ");    DList * pDList = dlist;    /* find the final element */    while(pDList->next)        pDList = pDList->next;    while(pDList->prev)    {        printf("%3d  ", pDList->value);        pDList = pDList->prev;    }    printf("\n");}/* This implement of Doubly Linked List have the head */void doubly_linked_list_main_test(void){    DList * dlist = doubly_linked_list_init();    doubly_linked_list_insert(dlist, 1);        doubly_linked_list_insert(dlist, 2);        doubly_linked_list_insert(dlist, 3);        doubly_linked_list_insert(dlist, 4);    doubly_linked_list_insert(dlist, 5);        doubly_linked_list_insert(dlist, 6);        doubly_linked_list_insert(dlist, 7);        doubly_linked_list_insert(dlist, 8);        doubly_linked_list_insert(dlist, 9);        doubly_linked_list_insert(dlist, 10);       //  doubly_linked_list_delete_by_index(dlist, 0);    doubly_linked_list_delete_by_index(dlist, 5);    doubly_linked_list_traverse_fordward(dlist);    doubly_linked_list_traverse_backward(dlist);    printf("DList len is %d\n", dlist->length);}