这是学生成绩管理系统，本人刚刚学到结构体

Question

这个程序我用的方法是动态链表。我打了那个count函数，以学号查找对应学生信息，然后求出总分和平均分，查找的方法是和上一个del函数相同的，指针遍历。但是在运行过程中del函数的使用是正常的，count函数一输入学号敲了回车就引发了异常，异常是读取访问权限冲突。p1 是 nullptr。我不懂是什么意思，也不知道怎么解决，上csdn搜索了一些解决方法都不适用，希望有朋友可以为我解惑，感激不尽！

#include <iostream>
#include <iomanip>
using namespace std;
#define NULL 0
int n;
struct Student
{
    int number;
    char name[20];
    char sex;
    int age;
    int lesson;
    int escore;
    int mscore;
    int hscore;
    Student* Next;
};
void printmenu()
{
    cout << setw(77) << "|*********学生成绩管理系统*********|" << endl;

    cout << setw(77) << "|----------------------------------|" << endl;
    cout << setw(77) << "|             主菜单项             |" << endl;
    cout << setw(77) << "|----------------------------------|" << endl;
    cout << setw(77) << "|         1-- 录入学生信息         |" << endl;
    cout << setw(77) << "|         2-- 编辑学生信息         |" << endl;
    cout << setw(77) << "|         3-- 查询学生信息         |" << endl;
    cout << setw(77) << "|         4-- 统计学生信息         |" << endl;
    cout << setw(77) << "|         5-- 显示学生信息         |" << endl;
    cout << setw(77) << "|         6-- 删除学生信息         |" << endl;
    cout << setw(77) << "|         0-- 退出学生信息         |" << endl;
    cout << setw(53) << "请输入选项（0-6）:";
}
Student *input(void)
{
    Student* p1, *p2, *head;
    p1 = p2 = new Student;
    n = 0;
    cout << "请输入学生学号:" << endl;
    cin >> p1->number;
    cout << "请输入学生姓名:" << endl;
    cin >> p1->name;
    cout << "请输入学生性别:" << endl;
    cin >> p1->sex;
    cout << "请输入学生年龄:" << endl;
    cin >> p1->age;
    cout << "请输入学生班级:" << endl;
    cin >> p1->lesson;
    cout << "请输入学生英语成绩:" << endl;
    cin >> p1->escore;
    cout << "请输入学生数学成绩:" << endl;
    cin >> p1->mscore;
    cout << "请输入学生高数成绩:" << endl;
    cin >> p1->hscore;
    head = NULL;
    while (p1->number != 0)
    {
        cout << "添加成功！" << endl;
        cout << endl;
        n = n + 1;
        if (n == 1)head = p1;
        else p2->Next = p1;
        p2 = p1;
        p1 = new Student;
        cout << "请输入学生学号:" << endl;
        cin >> p1->number;
        if (p1->number == 0)
        {
            cout << "录入结束！" << endl;
            break;
        }
        cout << "请输入学生姓名:" << endl;
        cin >> p1->name;
        cout << "请输入学生性别:" << endl;
        cin >> p1->sex;
        cout << "请输入学生年龄:" << endl;
        cin >> p1->age;
        cout << "请输入学生班级:" << endl;
        cin >> p1->lesson;
        cout << "请输入学生英语成绩:" << endl;
        cin >> p1->escore;
        cout << "请输入学生数学成绩:" << endl;
        cin >> p1->mscore;
        cout << "请输入学生高数成绩:" << endl;
        cin    >> p1->hscore;
    }
     p2->Next = NULL;
     return(head);
}
void print(Student *head)
{
    Student* p;
    
    p = head;
    if (head != NULL)
    {
        cout << "学号" << ' ' << "姓名" << ' ' << "性别" << ' ' << "年龄" << ' ' << "班级" << ' ' << "英语成绩" << ' ' << "数学成绩" << ' ' << "高数成绩" << endl;
        while (p != NULL)
        {
            cout << p->number << ' ' << p->name << ' ' << p->sex << ' ' << p->age << ' ' << p->lesson << ' ' << p->escore << ' ' << p->mscore << ' ' << p->hscore << endl;
            p = p->Next;
        }
    }
    else cout << "无记录!" << endl;
}
Student* del(Student* head, int num)
{
    Student* p1, *p2=0;
    if (head == NULL)
    {
        cout << "无记录！" << endl; 
        return(head);
    }
    p1 = head;
    while (num != p1->number && p1->Next != NULL)
    {
        p2 = p1;
        p1 = p1->Next;
    }
    if (num == p1->number)
    {
        if (p1 == head)
        {
           head = p1->Next;
        }
        else p2->Next = p1->Next;
        cout << "删除成功！" << endl;
    }
    else cout << "找不到该学号！" << endl;
    return(head);
}
void count(Student *head,int num)
{
    Student* p1=head, * p2=0;
    if (head == NULL)
    {
        cout << "无记录！" << endl;
    }
     
    while (p1->Next!=NULL&&num != p1->number)
    {
        p2 = p1;
        p1 = p1->Next;
    }
    
    if (p1->number == num)
    {
        cout << "该学生总分为:" << p1->escore + p1->mscore + p1->hscore << endl;
        cout << "该学生平均分为:" << (p1->escore + p1->mscore + p1->hscore) / 3 << endl;
    }
    else cout << "查无此人！" << endl;
}
int main()
{
    int a,b,num;
    Student* head=0,*newhead=NULL;
    
    while (1)
    {
        printmenu();
         cin >> a;
         switch (a)
         {
         case 0:
         {
             system("cls");
             cout << "感谢使用本程序!" << endl;
             return 0;
         }break;
         case 1:
             {
                 system("cls");
                 cout << "请录入学生信息！(若录入完成请在输入学号时输入0)" << endl;
                 head=input();
                 system("pause");
                 system("cls");
             }break;
         case 5:
         {
             system("cls");
             if (newhead != head)
             {
                 print(newhead);
             }
             else print(head);
             system("pause");
             system("cls");
         }break;
         case 6:
         {
             system("cls");
             cout << "请输入需要删除学生信息的学号" << endl;
             cin >> num;
              
             if (newhead != head)
             {
                newhead= del(newhead, num);
             }
             else del(head, num);
             system("pause");
             system("cls");
         }break;
         case 4:
         {
             system("cls");
             cout << "请输入要统计的学生的学号" << endl;
             cin >> num;
             if (newhead != head)
             {
                 count(newhead, num);
             }
             else count(head, num);
             system("pause");
             system("cls");
         }
         }
    }
            
         
}

```