c双向循环链表求指导

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

struct student                           //结构体创建
{
    char name[10];
    int score;
    struct student *prior;
    struct student *next;
   
};
struct student *insert(struct student *head)
{
     struct student *p,*pnew,*pold;
     pnew=(struct student *)malloc(sizeof(struct student));
     scanf("%s%d",pnew->name,&pnew->score);
     p=head;
      if(pnew->score>head->score){
         pnew->next=head;
         head->prior=pnew;
         head=pnew;
      }
      else{
          while(p!=NULL&&pnew->score<p->score){
            
          }
          pnew->next=p;
          p->prior=pnew;
          pold->next=pnew;
          pnew->prior=pold;
      }
      return head;
}
struct student *create(int n)
{
    struct student *head,*pnew,*ptail;
    int i;
    pnew=(struct student *)malloc(sizeof(struct student));
    printf("请输入学生姓名及成绩\n");
    scanf("%s%d",pnew->name,&pnew->score);
    head=ptail=pnew;                              //建立头节点
    for(i=1;i<n;i++){
          printf("请输入学生姓名及成绩\n");
          head=insert(head);
          }
    ptail->next=NULL;
    return head;
}
void print1(struct student *head)
{
    struct student *p=head;
    while(p!=NULL){
        printf("%s %f\n",p->name,p->score);
        p=p->next;
    }
}
void print2(struct student *head)
{
    struct student *p=head;
    while(p!=head){
        printf("%s %f",p->name,p->score);
        p=p->prior;
    }
    printf("%s %d\n",p->name,p->score);
}
struct student *pdelete(struct student *head,int grade)
{
    struct student *p,*pold,*ptail;
    p=head;
    while(head!=NULL&&head->score>=grade){
        head=head->next;
        p=head->prior;
        free(p);
        p=head;
    }
    if(head==NULL) return head;
    p=head->next;
    pold=head;
    ptail=p->next;
    while(head!=NULL){
        if(p->score>=grade){
           pold->next=p->next;
           pold=ptail->prior;
           free(p);
           p=pold->next;
        }
        else{
            pold=p;
            p=p->next;
            ptail->prior=pold;
        }
    }
    return head;
}
int destory(struct student *head)
{
    struct student *p,*q;
    while(p!=q)
    {
        p=head->prior;
        q=head->next;
        q->prior=q;
        p->next=q;
        free(head);
        head=q;
    }
    return 6;
}

void main()
{
    struct student *create(int n);
    void print1(struct student *head);
    void print2(struct student *head);
    struct student *insert(struct student *head);
    struct student *pdelete(struct student *head,int grade);
    int destory(struct student *head);               //函数声明

    int j,k=0,m,n;
    struct student *head;
    printf("请输入要建立节点的个数:\n");
    scanf("%d",&n);
    create(n);                                     //建立一链表并使其按成绩从高到地排列
    printf("请选择要进行的操作:\n1.插入;\n2.删除;\n3.正向遍历输出;\n4.反向遍历输出;\n5.销毁\n");
    scanf("%d",&m);
    while(k!=6){
    switch(m){
    case 1: printf("请输入要插入的学生姓名及成绩:\n");
            head=insert(head);
            print1(head);
            break;
    case 2: printf("请输入及格线");                 //删除已经及格的学生
            scanf("%d",&j);
            head=pdelete(head,j);
            print1(head);
            break;
    case 3:print1(head);                           //正向遍历输出
           break;
    case 4:print2(head);                           //反向遍历输出
           break;
    case 5:k=destory(head);                        //销毁
           break;
   
    }
    }
}