C语言--双链表

还是《程序员面试宝典》上的代码

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

typedef struct student
{
    int data;
    struct student *next;
    struct student *pre;
}dnode;

// 建立双链表
dnode *create()
{
    dnode *head,*p,*s;
    int x,cycle = 1;
    head = (dnode*)malloc(sizeof(dnode));
    p = head;
    while(cycle)
    {
        printf("\nPlease input the data:");
        scanf("%d",&x);
        if(x != 0)
        {
            s = (dnode*)malloc(sizeof(dnode));
            s->data = x;
            printf("%d\n",s->data);
            p->next = s;
            s->pre = p;
            p = s;
        }
        else
            cycle = 0;
    }

    p->next = NULL;

    p = head;
    head = head->next;
    head->pre = NULL;

    free(p); // 释放掉没有存数据的第一个节点


    printf("\nHead data:%d\n",head->data);
    return head;
}

// 求双链表长度，跟单链表一样
int length(dnode *head)
{
    int n = 0;
    dnode *p;
    p = head;
    while(p != NULL)
    {
        p = p->next;
        n++;
    }
    return n;
}

// 输出双链表，跟单链表一样
void print(dnode *head)
{
    dnode *p;
    int n;
    n = length(head);
    printf("\nNow,These %d records are:\n",n);
    p = head;
    printf("NuLL <=> ");
    while(p != NULL)
    {
        printf("%d <=> ",p->data);
        p = p->next;
    }
    printf("NULL\n");
}

// 双链表删除节点
dnode *del(dnode *head, int num)
{
    dnode *p1,*p2;
    p1 = head;
    while(num != p1->data && p1->next != NULL)
        p1 = p1->next;
    if(num == p1->data)
    {
        if(p1 == head)             //删除的是链表头
        {
            head = head->next;
            head->pre = NULL;
        }
        else if(p1->next == NULL)  //删除的是链表尾
        {
            p1->pre->next = NULL;
        }
        else                       // 删除的是链表中
        {
            p1->next->pre = p1->pre;
            p1->pre->next = p1->next;
        }
        free(p1);
    }
    else    // 没有找到要删除的节点
        printf("\n%d could not been found!",num);

    return head;
}


// 双链表插入节点

dnode *insert(dnode *head, int num)
{
    dnode *p0,*p1;
    p1 = head;
    p0 = (dnode *)malloc(sizeof(dnode));
    p0->data = num;

    while(p0->data > p1->data && p1->next != NULL)
        p1 = p1->next;
    if(p0->data <= p1->data)
    {
        if(head == p1)  //插入链表头前
        {
            p0->pre = NULL;
            p0->next = p1;
            p1->pre = p0;
            head = p0;
        }
        else            // 正常插入
        {
            p1->pre->next = p0;
            p0->next  =p1;
            p0->pre = p1->pre;
            p1->pre = p0;
        }
    }
    else               // num比链表中任何数都大，插入链表尾
    {
        p1->next = p0;
        p0->pre = p1;
        p0->next = NULL;
    }

    return head;
}


int main()
{
    dnode *stu_link;
    int n,del_num,insert_num;

    stu_link = create();
    print(stu_link);

    printf("\nPlease input a del num:");
    scanf("%d",&del_num);
    stu_link = del(stu_link,del_num);
    print(stu_link);

    printf("\nPlease input a insert num:");
    scanf("%d",&insert_num);
    stu_link = insert(stu_link,insert_num);
    print(stu_link);

    return 0;
}

可能链表输出的样子有点问题，就这样看吧
只有一个节点的时候执行删除操作会崩溃，，