链表读取访问权限冲突

刚开始学链表，写了一个非递减单链表，先po程序吧

#include<iostream>
using namespace std;
struct List
    //构建构建新节点的结构体
    //非递减顺序排列
{
    int data;
    List * next;
};

void insert(List * & headnode, int num)
{
        List *s, *p = headnode, *q;   //*s用于建立新节点 *p用于跟踪最新结点
        //以下三行为构建新结点
        s = new List;
        s->data = num;
        s->next = NULL;

        //以下if语句用于插入第一个结点
        if (p->next == NULL)
        {
            headnode->next = s;
            p = s;
            return;
        }

        //新节点数据最小时的操作
        if (headnode->next->data > s->data)
        {
            s->next = headnode->next;
            headnode->next = s;
            return;
        }

        //常规对比插入排序
        for (q = headnode->next, p = headnode->next->next; p; q = p, p = p->next)
        {
            if (p->data >= s->data)
            {
                s->next = p;
                q->next = s;
                return;
            }
        }
        q->next = s;
        return;
    }

void ShowList(const List * headnode)
{
    cout << "您所输入的数为：";
    while (headnode)
    {
        cout << headnode->data << '\t';
        headnode = headnode->next;
    }
    cout << endl;
}
int main()
{
    int k = 0, m = 0;  //k用于输入新数据 m用于计算节点数
    List * headnode = NULL;
    cout << "请输入除零以外的任何数" << endl;
    cin >> k;
    while (k != 0)
    {
        insert(headnode, k);
        cin >> k;
        m++;
        headnode->data = m;
    }
    ShowList(headnode);  //输出链表
}

然后再po出错的截图~

# 先在此谢过大神了！~

insert函数里有点问题，帮楼主改掉了，现在应该没错了，链表既然是没有头结点的，就没有必要从->next开始

 #include<iostream>
using namespace std;
struct List
    //构建构建新节点的结构体
    //非递减顺序排列
{
    int data;
    List * next;
};

void insert(List * & headnode, int num)
{
    List *s, *p = headnode, *q;   //*s用于建立新节点 *p用于跟踪最新结点
                                  //以下三行为构建新结点
    s = new List;
    s->data = num;
    s->next = NULL;

    //以下if语句用于插入第一个结点
    if (p == NULL)
    {
        headnode = s;
        //p = s;
        return;
    }

    //新节点数据最小时的操作
    if (headnode->data > s->data)
    {
        s->next = headnode;
        headnode = s;
        return;
    }

    //常规对比插入排序
    for (q = headnode, p = headnode->next; p; q = p, p = p->next)
    {
        if (p->data >= s->data)
        {
            s->next = p;
            q->next = s;
            return;
        }
    }
    q->next = s;
    return;
}

void ShowList(const List * headnode)
{
    cout << "您所输入的数为：";
    while (headnode)
    {
        cout << headnode->data << '\t';
        headnode = headnode->next;
    }
    cout << endl;
}
int main()
{
    int k = 0, m = 0;  //k用于输入新数据 m用于计算节点数
    List * headnode = NULL;
    cout << "请输入除零以外的任何数" << endl;
    cin >> k;
    while (k != 0)
    {
        insert(headnode, k);
        cin >> k;
        //m++;
        //headnode->data = m;
    }
    ShowList(headnode);  //输出链表
}

List * headnode = NULL;
然后
insert(headnode, k);
此时headnode是null

*p = headnode
if (p->next == NULL)
显然p是null怎么访问next