求助大佬，创建一个没有重复元素的单链表，出现错误怎么改呢

#include <iostream>
using namespace std;
template <class T>
struct node
{
    T data;
    node<T>* next;
};
template <class T>
class link
{
    node<T>* first;
public:
    link()
    {
        first = new node<T>;
        first->next = NULL;
    }

    link(T a[], int length)
    {
        first = new node<T>;
        first->next = NULL;

        node<T>* r = first;
        node<T>* w = new node<T>;//先插入第一个
        w->data = a[0];
        r->next = w;//插入的结点放在first之后
        r = w;//r指向查插入的第一个结点

        for (int i = 1; i < length; i++)
        {
            int flag = 0;
            node<T>* s=first->next;
            while (s != NULL)//寻找是否已经有相同的元素
            {
                if (s->data == a[i])
                {
                    flag = 1;
                    break;
                }
                s = s->next;
            }
            if (flag == 0)//没有相同的元素，将数组目前元素尾插法插入链表
            {
                node<T>* p = new node<T>;
                p->data = a[i];
                r->next = p;
                r= p;
            }
        }
        r->next = NULL;//最后将最后一个结点的next域置空

    }
    void show()
    {
        node<T> * p = first->next;
        while (p != nullptr)
        {
            cout << p->data << " ";
            p = p->next;
        }
    }
};

int main()
{
    int a[100];
    int m;
    cin >> m;
    for (int i = 0; i < m; i++)
    {
        cin >> a[i];
    }
    link<int> link2(a, m);
    link2.show();
    return 0;

}