How to fix LNK 2019 error

#pragma once
// file Node.h
#pragma once


class CNode
{
	friend class CDanhSachDinh;
	
public:
	CNode(const int&);
	~CNode(void);	
	int& GetInfo();
	

private:
	int _info;
	CNode *_pNext;
};

// file DanhSachDinh.h (DanhSachDinh in English means Vertice_List)
#pragma once
#include "Node.h"
#include <iostream>
#include <string>
using namespace std;

class CDanhSachDinh
{	
public:	
	CDanhSachDinh();
	CDanhSachDinh(const CDanhSachDinh&);
	~CDanhSachDinh();
	bool IsEmpty() const;
	int Length() const;
	bool InsertAt(const int&, int);
	void Append(const int&);
	void Delete();
	CNode* GetHead();
	int& ElementAt(int);
	void Input();
	friend ostream & operator << (ostream &, CDanhSachDinh &);
	friend istream & operator >> (istream &, CDanhSachDinh &); 
	
private:
	CNode* _pHead;
	CNode* _pTail;
	int _length;
	std::string _result;
	void Print();
};

// file Node.cpp
#include "Node.h"

CNode::CNode(const int& value)
{
	this->_info = value;
	this->_pNext = NULL;
}

CNode::~CNode(void)
{
}

int& CNode::GetInfo()
{
	return this->_info;
}

// file DanhSachDinh.cpp (DanhSachDinh in English means Vertice_List)

#include "DanhSachDinh.h"
#include "Node.h"
#include <cstdio>
#include <cstdlib>
#include <iostream>
#include <string>

CDanhSachDinh::CDanhSachDinh(void)
{
	this->_pHead = NULL;
	this->_pTail = NULL;
	this->_length = 0;
	this->_result = "";
}

CDanhSachDinh::CDanhSachDinh(const CDanhSachDinh& list)
{
	if(list.IsEmpty())
	{
		this->_pHead = this->_pTail = NULL;
		this->_length = 0;
	}
	else
	{
		this->_pHead = new CNode(list._pHead->_info);
		this->_pTail = this->_pHead;
		CNode *p;
		for(p = list._pHead->_pNext; p; p = p->_pNext)
			this->Append(p->_info);
		this->_length = list._length;
	}
}

CDanhSachDinh::~CDanhSachDinh()
{

}

CNode* CDanhSachDinh::GetHead()
{
	return NULL;
}

void CDanhSachDinh::Append(const int& x)
{
	CNode *p = new CNode(x);
	if(this->IsEmpty())	// Danh sách rỗng
	{
		this->_pHead = this->_pTail = p;
	}
	else	// Danh sách có ít nhất 1 phần tử
	{
		this->_pTail->_pNext = p;
		this->_pTail = p;
	}
	this->_length++;
}

bool CDanhSachDinh::InsertAt(const int& data, int position)
{
	// Vị trí chèn x vào nằm cuối cùng
	if(position == this->_length + 1)
	{
		this->Append(data);
		return true;
	}

	// Vị trí không hợp lệ
	if(!(position >= 1 && position <= this->_length))
		return false;

	// Chèn x vào
	CNode *newp = new CNode(data);
	if(position == 1)
	{
		newp->_pNext = this->_pHead;
		this->_pHead = newp;
	}
	else
	{
		CNode *p = this->_pHead;
		while(position > 2)
		{
			p = p->_pNext;
			position--;
		}
		newp->_pNext = p->_pNext;
		p->_pNext = newp;
	}	
	return true;
}

bool CDanhSachDinh::IsEmpty() const
{
	return this->_length==0;
}

void CDanhSachDinh::Print()
{	
	char buffer[30];
	std::string result = "";	
	result += "{";
	CNode* p = this->_pHead;
	while(p)
	{
		_itoa_s(p->_info, buffer, 10);
		result.append(buffer);
		result.append("; ");
		p = p->_pNext;
		
	}
	result += "}";
	//cout<<result<<endl;
	this->_result = result;
	
}

void CDanhSachDinh::Input()
{
	int n;
	cout<<" - Nhap bao nhieu dinh: ";
	cin>>n;
	for(int i = 1; i <= n; i++)
	{
		int x;
		cout<<" - Nhap phan tu thu "<<i<<" = ";
		cin>>x;
		this->Append(x);
	}
}

int CDanhSachDinh::Length() const
{
	return this->_length;
}

ostream& operator << (ostream & os, CDanhSachDinh & list)
{
	list.Print();
	os << list._result << endl;
	return os; 
}