计算linkedlist元素的标准差

Question

我想计算着墨列表元素的标准偏差，所以我写了一个函数来计算平均值和总和，然后用公式计算标准差，但是我得到了错误＆＃34; 0xC0000005：访问冲突读取位置0x00000000。＆＃34;在第46行，

sd += ((first->info)-average)*((first->info-average);

为什么会出现这个问题，有没有办法摆脱它？ 我的代码如下：

class Node
{
public:
    int info;
    Node* next;
        int value;
};
class List:public Node
{
    Node *first,*last;
public:
    List()
    {
        first=NULL;
        last=NULL;
    }
    void create();
    void insert();
    void delet();
    void display();
    void search();
    void sum();
    void sd();
};
void List::sd()
{
    system("cls");
    cout<<"The Linked List Operations Program";
        cout<<"\n==========================================================="<<endl;
    Node *temp=first;
      int sd = 0;
     int length=0;
    while(temp!=NULL)
    {
        length++;
        temp=temp->next;
    }
        int sum = 0;
       while (first != NULL) 
    {
     sum += first->info; 
     first = first->next;
    }
   int average;
   average = (sum/length);
   sd += ((first->info)-average)*((first->info-average);
   first=first->next;
   cout<<average;
   cout<<"The standard diviation is  " <<sqrt(sd)/10 ;
   cin.get();
   cin.get();
}
int main()
{
    system("cls");
    int m;
    List l;
    Node *first;
    int ch;
    l.sd();
    return 0 ; 
}
void List::create()
{
    system("cls");
    cout<<"The Linked List Operations Program";
        cout<<"\n==========================================================="<<endl;
    Node *temp;
    temp=new Node;
    int n;
    cout<<"\nEnter an element to create the first node of the linkedlist:";
    cin>>n;
    temp->info=n;
    temp->next=NULL;
    if(first==NULL)
    {
        first=temp;
        last=first;
    }

    else
    {
        last->next=temp;
        last=temp;
    }
}
void List::insert()
{
        system("cls");
        cout<<"The Linked List Operations Program";
        cout<<"\n==========================================================="<<endl;
    Node *prev,*cur;
    prev=NULL;
    cur=first;
    int count=1,pos,ch,n;
    Node *temp=new Node;
    cout<<endl<<"ADDED";
    system("cls");
    temp->info=rand();
    temp->next=NULL;
    last->next=temp;
    last=temp;  

}