【链表】带头结点的双向循环链表

还需改进：

creat_node这个函数应有返回类型，来判断新建结点是否成功，不然主函数中不管成不成功都会访问该节点成员。

改了这个函数，在主函数中create_node后要判断是否成功，不成功就提示并退出函数，退出前别忘了还要释放链表！

同时create_link这个函数中也要判断head是否申请成功，不成功的话同样提示并退出函数。

#include <stdio.h>
#include <stdlib.h>

struct dnode
{
    int num;

    struct dnode * prior;
    struct dnode * next;
};

typedef struct dnode Dnode;
typedef struct dnode * Dlink;

void create_dnode(Dlink *DblNode)
{
    int count = 10;

    do
    {
        *DblNode = (Dlink)malloc(sizeof(Dnode));
        count--;
    }while(!is_malloc_ok(*DblNode) && count);
}

int is_malloc_ok(Dlink DblNode)
{
    if(DblNode == NULL)
    {
        printf("malloc error!\n");
        return 0;
    }
    return 1;
}

void create_dlink(Dlink *DblHead)
{
    Dlink DblNode;
    create_dnode(&DblNode);

    *DblHead = DblNode;
    DblNode->next = DblNode;
    DblNode->prior= DblNode;
}

void insert_dnode_head(Dlink DblHead,Dlink DblNode)
{
    DblHead->next->prior = DblNode;
    DblNode->next = DblHead->next;
    DblHead->next = DblNode;
    DblNode->prior = DblHead;
}

void display(Dlink DblHead)
{
    if(DblHead == NULL)
    {
        printf("该链表已被释放\n");
        return;
    }

    Dlink p = DblHead;
    
    printf("该链表显示是:\n");
    if(p->next == DblHead)
    {
        printf("Link is empty!\n");
    }
    else
    {
        p = p->next;
        while(p != DblHead)
        {
            printf("num is %d\n",p->num);
            p = p->next;
        }
    }
}

int length(Dlink DblHead)
{
    Dlink p = DblHead->next;
    int count = 0;

    while(p != DblHead)
    {
        count++;
        p = p->next;
    }

    return count;
}

void insert_dnode_tail(Dlink DblHead,Dlink DblNode)
{
    DblNode->prior = DblHead->prior;
    DblHead->prior->next = DblNode;
    DblNode->next = DblHead;
    DblHead->prior = DblNode;
}

void insert_dnode_mid_before(Dlink DblHead,Dlink DblNode,int num)
{
    Dlink p = DblHead->next;

    while(p != DblHead && p->num != num)
    {
        p = p->next;
    }

    if(p == DblHead)
    {
        printf("No such node!\n");
    }
    else
    {
        DblNode->prior = p->prior;
        p->prior->next = DblNode;
        DblNode->next = p;
        p->prior = DblNode;
    }
}

void insert_dnode_mid_after(Dlink DblHead,Dlink DblNode,int num)
{
    Dlink p = DblHead->next;

    while(p != DblHead && p->num != num)
    {
        p = p->next;
    }

    if(p == DblHead)
    {
        printf("No such node!\n");
    }
    else
    {
        p->next->prior = DblNode;
        DblNode->next = p->next;
        p->next = DblNode;
        DblNode->prior = p;
    }
}

void delete_dnode(Dlink DblHead,int num)
{
    Dlink p = DblHead->next;

    while(p != DblHead && p->num != num)
    {
        p = p->next;
    }

    if(p == DblHead)
    {
        printf("No such node!\n");
    }
    else
    {
        p->next->prior = p->prior;
        p->prior->next = p->next;
        free(p);
    }
}

Dlink find(Dlink DblHead,int num)
{
    Dlink p = DblHead->next;

    while(p != DblHead && p->num != num)
    {
        p = p->next;
    }

    if(p == DblHead)
    {
        p = NULL;
        return p;
    }
    else
    {
        return p;
    }
}

void make_empty(Dlink DblHead)
{
    Dlink p = DblHead->next;
    Dlink q;

    while(p != DblHead)
    {
        q = p->next;
        p->next->prior = p->prior;
        p->prior->next = p->next;
        free(p);
        p = q;
    }
}

void release_dlink(Dlink *DblHead)
{
    make_empty(*DblHead);
    free(*DblHead);
    *DblHead = NULL;
}

int main()
{
    Dlink DblHead;
    Dlink DblNode;
    int i;
    int num;

    create_dlink(&DblHead);

    for(i = 0 ;i < 5; i++)
    {
        create_dnode(&DblNode);
        DblNode->num = i + 1;

        //insert_dnode_head(DblHead,DblNode);
        insert_dnode_tail(DblHead,DblNode);
    }

    display(DblHead);
    printf("该链表长度为%d\n\n",length(DblHead));


    create_dnode(&DblNode);
    printf("请输入你想插入的结点的数值:");
    scanf("%d",&DblNode->num);
    printf("请输入你想在值为哪个的结点前插入该结点:");
    scanf("%d",&num);

    insert_dnode_mid_before(DblHead,DblNode,num);

    display(DblHead);
    printf("该链表长度为%d\n\n",length(DblHead));


    create_dnode(&DblNode);
    printf("请输入你想插入的结点的数值:");
    scanf("%d",&DblNode->num);
    printf("请输入你想在值为哪个的结点后插入该结点:");
    scanf("%d",&num);

    insert_dnode_mid_after(DblHead,DblNode,num);

    display(DblHead);
    printf("该链表长度为%d\n\n",length(DblHead));

    
    printf("请输入你想删除的结点的数值:");
    scanf("%d",&num);

    delete_dnode(DblHead,num);
    
    display(DblHead);
    printf("该链表长度为%d\n\n",length(DblHead));

    
    printf("请输入你想查找的结点的数值:");
    scanf("%d",&num);

    DblNode = find(DblHead,num);
    if(DblNode == NULL)
    {
        printf("没有这个结点\n");
    }
    else
    {
        printf("这个结点的数值是%d\n\n",DblNode->num);
    }

    make_empty(DblHead);

    display(DblHead);
    printf("该链表长度为%d\n\n",length(DblHead));

    release_dlink(&DblHead);
    display(DblHead);
    
    

    return 0;
}