刚刚学习数据结构，写了个链表的基本程序，到处都是错，求大虾指教（真心想学啊）

#include<iostream>
#include<stdlib.h>
using namespace std;
class LinkNode{
public:
    int data;
    LinkNode *link;
    LinkNode(int x){data = x;}
    LinkNode(){}
};
class List{
private:
    LinkNode *first;
public:
    bool Insert(int i,int x);           //插入一个节点
    void inputRear();                   //建立链表
    void output();                      // 输出
    List(){first = new LinkNode;}
    virtual ~List(){
        if (first !=NULL)
        {
            LinkNode *cur = first->link;
            delete first;
            first = cur;

        }
    }
};
bool List::Insert(int i,int x){
    if(first==NULL || i==0){                 //表头      
        LinkNode *newNode=new LinkNode(x);
        if(newNode==NULL){cout<<"No\n";exit(1);}        
        newNode->link=first;first=newNode;        
    }
    else {                                 
        LinkNode *current=first;
        for(int k=1;k<i;k++){
            if(current==NULL) break;
            else current=current->link;
        }
        if(current==NULL && first!=NULL){cout<<"No\n";return false;}
        else{
            LinkNode *newNode=new LinkNode(x);
            if(newNode==NULL){cout<<"No\n";exit(1);}
            newNode->link=current->link;
            current->link=newNode;
        }
    }
    return true;
};
void List::inputRear(){
    LinkNode *last;
    int val;
    //LinkNode *newNode=new LinkNode;   //创建一个新节点
    last = first;   
    //if(first==NULL){cout <<"NO";exit(1);}
    cout<<"请输入链表数据：";
    cin>>val;
    while(val!='\n'){
        LinkNode*newNode=new LinkNode(val);
        last->link=newNode;
        last=newNode;
        cout<<"请输入链表数据：";
        cin>>val;
    }
};
void List::output(){
    LinkNode *current1=first->link;
    while(current1!=NULL){
        cout<<current1->data<<endl;
        current1=current1->link;
    }
};
int main(){
    List A;
    A.inputRear();
    A.output();
    A.Insert(3, 5);
    A.output();
    system("pause");
    return 0;
}

我错在哪里啊