双向循环链表相关操作

本实验主要完成双向循环链表的
创建
添加数据
查找数据
删除数据
遍历
实验过程中遇到的问题：
1、malloc空间申请
2、malloc空间释放
问题1
函数129行处，在输入学生姓名之前为姓名申请了20个字符的空间；可以看到，链表的数据域是一个结构体，结构体包含学生的三个属性信息；
在实现的过程中，将姓名申请为字符指针用于存储学生姓名；因此除了在添加节点时为节点Node申请空间外，还需要为姓名申请空间（如果将姓名指定为字符数组将不涉及这个问题）；
问题2
空间释放问题
函数169行处，在删除节点之后，我们需要释放节点申请的空间（Node）；同时也需要将存放名字的空间释放掉，但是问题是，我们需要函数返回姓名，如果在del函数中将空间释放掉，我们将无法获取姓名，所以本文在调用完del函数后在主函数总释放了姓名的空间；

关于双向循环链表：
1、头结点

作为头结点，数据域为空，前向指针和后向指针都指向自己；
2、其他节点

如图这样三个节点Node是头结点，其他两个是数据节点；
假设现在需要删除节点Node1；
需要先将图中的四条线断开（×号标注）

断开后会发现，Node2的prior和Node的next没有指向的节点，此时需要将Node2的prior指向Node，Node的next指向Node2；

节点的增加也是这样，先断后连，但是需要注意的是断和连的先后关系，有些时候需要中间变量；注意这样的逻辑就不会出错。

#include<stdio.h>#define ERROR -1#define OK 0#define NOTEXIST 1typedef int State;struct studentInfo{    int id;    int age;    char *name;}studentInfo;struct Node{    struct studentInfo student;    struct Node *prior;    struct Node *next;}Node;typedef struct Node * LinkList;typedef struct studentInfo * Student;int system(const char *string);//创建链表State createLinkList(LinkList *L) {    (*L) = (LinkList)malloc(sizeof(Node));    if ((*L) == NULL)        return ERROR;    (*L)->next = (*L);    (*L)->prior = (*L);    return OK;}//向链表中添加数据//链表中添加数据有两种方式//一种头插法一种尾插法//尾插法需要寻找链表尾部State addDataFirst(LinkList *L, struct studentInfo student) {    LinkList p;    p = (LinkList)malloc(sizeof(Node));    p->student = student;    p->next = (*L)->next;    (*L)->next->prior = p;    (*L)->next = p;    p->prior = (*L);//  printf("%d,%d,%s\n", student.id, student.age, student.name);    return OK;}State addDataLast(LinkList *L, struct studentInfo student) {    LinkList p;    p = (LinkList)malloc(sizeof(Node));    p->student = student;    (*L)->prior->next = p;    p->prior = (*L)->prior;    (*L)->prior = p;    p->next = (*L);//  printf("%d,%d,%s\n", student.id, student.age, student.name);    return OK;}//数据查找//感兴趣的朋友可以多写几个查找函数，完成按照姓名，学号，//年龄等不同属性查找的功能。State findData(LinkList L, int id, Student findStu) {    LinkList p;    p = L->next;    while ((p->student).id != id)    {        if (p->next == L) {            return NOTEXIST;        }        p = p->next;    }    *findStu = (p->student);    return OK;}//数据删除State deleData(LinkList *L, int id, Student studentPtr) {    LinkList p;    p = (*L)->next;    while ((p->student).id != id)    {        if (p->next == L) {            return NOTEXIST;        }        p = p->next;    }    p->next->prior = p->prior;    p->prior->next = p->next;    (*studentPtr) = p->student;    free(p);    return OK;}//数据遍历State printData(LinkList L) {    LinkList p;    struct studentInfo student;    p = L->next;    if (p == L)        return NOTEXIST;    while (p != L) {        student = p->student;        printf("%d,%d,%s\n", student.id, student.age, student.name);        p = p->next;    }    return OK;}int main() {    struct studentInfo student;    LinkList L;    LinkList p;    State state = 0;    int option =1;    char flag = 0;    int id;    createLinkList(&L);    printf("Add student information(0)\n");    printf("Find the student information by id(1)\n");    printf("Delete the student information by id(2)\n");    printf("Check all students information(3)\n");    printf("Quite(4)\n");    while (1) {        printf("please input the option!\n");        scanf("%d", &option);        system("cls");        if(option>4 || option <0 ) {            printf("Don't have this option!\n");            printf("please resume load!\n");        }        while (option == 0) {            printf("Add student information is:\n");            student.name = (char *)malloc(20 * sizeof(char));            scanf("%d,%d,%s", &student.id, &student.age, student.name);            state = addDataLast(&L, student);            if (!state)                printf("Add student sucessed!\n");            printf("press[ENTER]continue!press others quite...\n");            flag=getchar();            flag = getchar();            if (flag != '\n')                break;        }        while (option == 1) {            printf("Find the student by id(1)\n");            printf("please input id:\n");            scanf("%d", &id);            state = findData(L, id, &student);            if (state == OK) {                printf("Find sucessed!\nThe information is:\n");                printf("%d,%d,%s\n", student.id, student.age, student.name);            }            else {                printf("This id don't exit!\n");                printf("All student is:\n");                printData(L);                printf("please input id agin!\n");            }            printf("press[ENTER]continue!press others quite...\n");            flag = getchar();            flag = getchar();            if (flag != '\n')                break;        }        while(option == 2) {            printf("Delete the student information by id(2)\n");            printf("please input id\n");            scanf("%d", &id);            state = deleData(&L, id, &student);            if (state == OK) {                printf("Delete sucessed!\nThe information is :\n");                printf("%d,%d,%s\n", student.id, student.age, student.name);                free(student.name);            }            else {                printf("This id don't exit!\n");                printf("All student is:\n");                printData(L);                printf("please input id agin!\n");            }            printf("press[ENTER]continue!press others quite...\n");            flag = getchar();            flag = getchar();            if (flag != '\n')                break;        }        if (option == 3) {            printf("All student is:\n");            printData(L);        }        else if (option == 4)            break;        else            ;    }    return OK;}