（带表头）双向链表的创建、插入、清空、删除

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

struct double_node
{
    int num;
    struct double_node * next;
    struct double_node * prior;
};

typedef struct double_node Dnode;
typedef struct double_node * Dlink;

int is_malloc_ok(Dlink new_node)
{
    if (new_node == NULL)
    {
        return 0;
    }
    else
    {
        return 1;
    }
}

void create_new_node(Dlink *new_node)
{
    int count = 10;

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

void create_link(Dlink *head)
{
    create_new_node(head);
    (*head)->next = (*head)->prior = *head;
}

void insert_node_head(Dlink head,Dlink new_node)
{
    new_node->prior = head;//新结点前驱指向头
    new_node->next = head->next;//新结点后继指向头结点的后继
    head->next = new_node;//头结点指向新结点
    new_node->next->prior = new_node;//将新结点放入下一个结点的前驱
}

void insert_node_tail(Dlink head,Dlink new_node)
{
    Dlink p;

    p = head;

    while (p->next != head)
    {
        p = p->next;
    }

    new_node->prior = p;//新结点的前驱指向尾结点
    new_node->next = head;//新结点的后继指向头结点
    p->next = new_node;//尾结点的后继指向新结点
    //new_node->next->prior = new_node;//新结点放入头结点的前驱
    head->prior = new_node;
}

void insert_node_mid_before(Dlink head,Dlink new_node,int loc)
{
    Dlink p, q;

    if (head->next == head)
    {
        printf("the link is empty!\n");
    }
    else
    {
        p = head->next;
        q = head;

        while (p->next != head && p->num != loc)
        {
            q = p;
            p = p->next;
        }

        if (p->next == head && p->num != loc)
        {
            printf("no such node!\n");
        }
        else
        {
            new_node->next = p;
            new_node->prior = q;
            q->next = new_node;
            new_node->next->prior = new_node;
        }
    }
}

void insert_node_mid_after(Dlink head,Dlink new_node,int loc)
{
    Dlink p;

    if (head->next == head)
    {
        printf("the link is empty!\n");
    }
    else
    {
        p = head->next;

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

        if (p->next == head && p->num != loc)
        {
            printf("no such node!\n");
        }
        else
        {
            new_node->next = p->next;
            new_node->prior = p;
            p->next = new_node;
            new_node->next->prior = new_node;
        }
    }
}

void delete(Dlink head,int num)
{
    Dlink p;

    if (head->next == head)
    {
        printf("the link is empty!\n");
    }
    else
    {
        p = head->next;

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

        if (p->next == head && p->num != num)
        {
            printf("no such node!\n");
        }
        else
        {
            p->next->prior = p->prior;
            p->prior->next = p->next;
            free(p);
        }
    }
}

void display(Dlink head)
{
    Dlink p;

    if (head == NULL)
    {
        printf("the link is not exist!\n");
    }
    else
    {
        //if (head->next == head && head->prior == head)
        if (head->prior == head)
        //if (head->next == head)
        {
            printf("the link is empty!\n");
        }
        else
        {
            p = head->next;
            while (p != head)
            {
                printf("num = %d\n",p->num);
                p = p->next;
            }
        }
    }
}

void make_empty(Dlink head)
{
    Dlink p;

    if (head == NULL)
    {
        printf("the link is not exist!\n");
    }
    else
    {
        //if (head->next == head && head->prior == head)
        if (head->prior == head)
        {
            printf("the link is empty!\n");
        }
        else
        {
            while (head->next != head)
            {
                p = head->next;
                head->next = p->next;
                p->next->prior = head;
                free(p);
            }
        }
    }
}

void release(Dlink *head)
{
    Dlink p;

    if (*head == NULL)
    {
        printf("the link is not exist!\n");
    }
    else
    {
        //while ((*head)->next != *head)
        while ((*head)->prior != *head)
        {
            p = (*head)->next;
            (*head)->next = p->next;
            p->next->prior = *head;
            free(p);
        }

        if ((*head)->next == *head)
        {
            free(*head);
            *head = NULL;
        }
    }
}

int main()
{
    Dlink head;
    Dlink new_node;
    int i, location;

    create_link(&head);

    for (i = 0; i < 10; i++)
    {
        create_new_node(&new_node);
        new_node->num = i + 1;
        //insert_node_head(head,new_node);
        insert_node_tail(head,new_node);
    }

    display(head);

    create_new_node(&new_node);

    printf("please input a num for insert :　");
    scanf("%d",&new_node->num);
    printf("please input location : ");
    scanf("%d",&location);

    //insert_node_mid_before(head,new_node,location);
    insert_node_mid_after(head,new_node,location);

    display(head);

    printf("please input a num for delete : ");
    scanf("%d",&location);

    delete(head,location);

    display(head);
    //make_empty(head);

    release(&head);
    release(&head);
    //display(head);

    return 0;
}