双向链表（含循环双链表）

双向链表
拓展1：前一节点的next可以看成是下一节点，某节点的pre可以看成是上一节点（偶尔会把next和pre看成一条线）
先讲头插法，图片解释

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尾插法，图片解释

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插入操作，图片解释
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删除操作，图片解释

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无头节点版双链表：

#define _CRT_SECURE_NO_WARNINGS
#include <iostream>
#include <stdlib.h>
using namespace std;
typedef struct node
{
    int data;
    struct node* pre;
    struct node* next;
}Node, * Link;
Link create()
{
    Link head = (Link)malloc(sizeof(Node));
    head->pre = NULL;
    head->next = NULL;
    return head;
}
void headadd(Link head, int newdata)
{
    Link s = (Link)malloc(sizeof(Node));
    s->data = newdata;
    s->next = head->next;
    s->pre = head;
    if (head->next == NULL)
    {
        head->next = s;
    }
    else
    {
        head->next->pre = s;
        head->next = s;
    }
}
void tailadd(Link head, int newdata)
{
    Link s = (Link)malloc(sizeof(Node));
    Link r = head;
    while (r->next)
    {
        r = r->next;
    }
    s->data = newdata;
    s->next = NULL;
    s->pre = r;
    r->next = s;
    r = s;
}
void insert(Link head,int i, int data)//i从head->next开始，i>=1
{
    Link s = (Link)malloc(sizeof(Node));
    Link p = head;
    s->data = data;
    for (int j = 1; j < i; j++)//找到插入位置i的前一个位置
    {
        p = p->next;
    }
    s->next = p->next;
    s->pre = p;
    p->next = s;
}
void del(Link head,int i)
{
    Link p = head;
    int j = 0;
    for (int j = 1; j < i; j++)//找到删除位置i的前一个位置
    {
        p = p->next;
    }
    if (p->next->next == NULL)
    {
        free(p->next);//与下面两句不要弄错顺序，因为弄错顺序的意思不一样，如果先指空的话，程序还是没有释放掉该节点空间
        p->next = NULL;
    }
    else
    {
        Link q = p->next;
        q->next->pre = p;
        p->next = q->next;
        free(q);
    }
}
void print(Link head)//打印和单链表一样，其实打印也就是比遍历多出一个cout那句
{
    Link p = head->next;
    while (p)
    {
        cout << p->data << " ";
        p = p->next;
    }
}
void destroy(Link head)//销毁和单链表一样
{
    Link p = head;
    Link q;
    while (p)
    {
        q = p->next;//先保留下一点的地址
        free(p);
        p = q;//此时p已经移动到q的位置，有点像继承遗产
    }
}
int main()
{
    Link head = create();
    int n0;//链表有值点个数
    printf("输入链表所需的有效长度：");
    scanf("%d", &n0);
    for (int i = 0; i < n0; i++)
    {
        tailadd(head, 2*i);
    }
    cout << "初始时链表为这样：";
    print(head);
    cout << endl;

    int n1;
    printf("要插入的位置n1为（有值点下标从1开始）：");
    scanf("%d", &n1);
    printf("要插入的元素n2为：");
    int n2;
    scanf("%d", &n2);
    insert(head, n1, n2);
    cout << "插入后链表为这样：";
    print(head);
    cout << endl;

    int n3;
    printf("要删除的位置n3（有值点下标从1开始）：");
    scanf("%d", &n3);
    cout << "删除后链表为这样：";
    del(head,n3);
    print(head);

    destroy(head);
    return 0;
}

有头节点版双链表，就是Link create()多了个L，大部分功能函数的head改为了L

#define _CRT_SECURE_NO_WARNINGS
#include <iostream>
#include <stdlib.h>
using namespace std;
typedef struct node
{
    int data;
    struct node* pre;
    struct node* next;
}Node, * Link;
Link create()
{
    Link head = (Link)malloc(sizeof(Node));
    Link L = (Link)malloc(sizeof(Node));
    head->next = L;
    L->pre = head;
    L->next = NULL;
    return head;
}
void headadd(Link L, int newdata)
{
    Link s = (Link)malloc(sizeof(Node));
    s->data = newdata;
    s->next = L->next;
    s->pre = L;
    if (L->next == NULL)
    {
        L->next = s;
    }
    else
    {
        L->next->pre = s;
        L->next = s;
    }
}
void tailadd(Link L, int newdata)
{
    Link s = (Link)malloc(sizeof(Node));
    Link r = L;
    while (r->next)
    {
        r = r->next;
    }
    s->data = newdata;
    s->next = NULL;
    s->pre = r;
    r->next = s;
    r = s;
}
void insert(Link head, int i, int data)//i从head->next开始，i>=1
{
    Link s = (Link)malloc(sizeof(Node));
    Link p = head;
    s->data = data;
    for (int j = 1; j < i; j++)//找到插入位置i的前一个位置
    {
        p = p->next;
    }
    s->next = p->next;
    s->pre = p;
    p->next = s;
}
void del(Link L, int i)
{
    Link p = L;
    int j = 0;
    for (int j = 1; j < i; j++)//找到删除位置i的前一个位置
    {
        p = p->next;
    }
    if (p->next->next == NULL)
    {
        free(p->next);//与下面两句不要弄错顺序，因为弄错顺序的意思不一样，如果先指空的话，程序还是没有释放掉该节点空间
        p->next = NULL;
    }
    else
    {
        Link q = p->next;
        q->next->pre = p;
        p->next = q->next;
        free(q);
    }
}
void print(Link L)//打印和单链表一样，其实打印也就是比遍历多出一个cout那句
{
    Link p = L->next;
    while (p)
    {
        cout << p->data << " ";
        p = p->next;
    }
}
void destroy(Link head)//销毁和单链表一样
{
    Link p = head;
    Link q;
    while (p)
    {
        q = p->next;//先保留下一点的地址
        free(p);
        p = q;//此时p已经移动到q的位置，有点像继承遗产
    }
}
int main()
{
    Link head = create();
    int n0;//链表有值点个数
    printf("输入链表所需的有效长度：");
    scanf("%d", &n0);
    for (int i = 0; i < n0; i++)
    {
        tailadd(head->next, 2 * i);
    }
    cout << "初始时链表为这样：";
    print(head->next);
    cout << endl;

    int n1;
    printf("要插入的位置n1为（有值点下标从1开始）：");
    scanf("%d", &n1);
    printf("要插入的元素n2为：");
    int n2;
    scanf("%d", &n2);
    insert(head->next, n1, n2);
    cout << "插入后链表为这样：";
    print(head->next);
    cout << endl;

    int n3;
    printf("要删除的位置n3（有值点下标从1开始）：");
    scanf("%d", &n3);
    cout << "删除后链表为这样：";
    del(head->next, n3);
    print(head->next);

    destroy(head->next);
    return 0;
}

循环双链表

#define _CRT_SECURE_NO_WARNINGS
#include <iostream>
#include <stdlib.h>
using namespace std;
typedef struct node
{
    int data;
    struct node* pre;
    struct node* next;
}Node, * Link;
Link create()
{
    Link head = (Link)malloc(sizeof(Node));
    head->pre = head;
    head->next = head;
    return head;
}
void tailadd(Link head)
{
    Link r = head;
    int flag = 1;
    int c;
    while (flag)
    {
        cin >> c;
        if (c != -99999)
        {
            Link s = (Link)malloc(sizeof(Node));
            s->data = c;
            s->next = head;
            s->pre = r;
            r->next = s;
            r = s;
        }
        else
        {
            flag = 0;
        }
    }
}
void headadd(Link head)
{
    int flag = 1;
    int c;
    while (flag)
    {
        cin >> c;
        if (c != -99999)
        {
            Link s = (Link)malloc(sizeof(Node));
            s->data = c;
            s->next = head->next;
            s->pre = head;
            if (head->next == NULL)
            {
                head->next = s;
            }
            else
            {
                head->next->pre = s;
                head->next = s;
            }
        }
        else
        {
            flag = 0;
        }
    }
}
void insert(Link head, int i, int data)//i从head->next开始，i>=1(同普通双链表)
{
    Link s = (Link)malloc(sizeof(Node));
    Link p = head;
    s->data = data;
    for (int j = 1; j < i; j++)//找到插入位置i的前一个位置
    {
        p = p->next;
    }
    s->next = p->next;
    s->pre = p;
    p->next = s;
}
void del(Link head, int i)
{
    Link p = head;
    int j = 0;
    for (int j = 1; j < i; j++)//找到删除位置i的前一个位置
    {
        p = p->next;
    }
    Link q = p->next;
    q->next->pre = p;
    p->next = q->next;
    free(q);
}
void print(Link head)//打印和单链表一样，其实打印也就是比遍历多出一个cout那句
{
    Link p = head->next;
    while (p!=head)
    {
        cout << p->data << " ";
        p = p->next;
    }
}
void destroy(Link head)//销毁和单链表有点不一样
{
    Link p = head->next;
    Link q;
    while (p!=head)
    {
        q = p->next;//先保留下一点的地址
        free(p);
        p = q;//此时p已经移动到q的位置，有点像继承遗产
    }
    free(head);
}
int main()
{
    cout << "开始初始化..............................................." << endl;
    Link head = create();
    cout << "初始化操作完毕！" << endl;
    cout << "开始建表，请输入元素：(这里是尾插法建表，输入-99999结束建表)..........." << endl;
    tailadd(head);
    cout << "建表操作完毕！" << endl;
    cout << "打印线性表中的所有数据：";
    print(head);
    cout << "-------------------------------------------------" << endl;
    cout << "开始插入（在第6个位置插入81）............................" << endl;
    insert(head, 6, 81);
    cout << "插入操作完毕！" << endl;
    cout << "打印线性表中的所有数据：";
    print(head);
    cout << "-------------------------------------------------" << endl;
    cout << "开始删除（这里删除第2个元素）............................" << endl;
    del(head, 2);
    cout << "删除操作完毕!" << endl;
    cout << "删除后打印线性表中的所有数据：";
    print(head);
    destroy(head);
    return 0;
}