Failed to debug

I am getting a "Debug failed" error when my program terminates. I've been trying to fix this for a long time and just can't seem to find the cause. Even my university professor said he saw nothing wrong. So, you are my last hope, stackoverllow. Please, help.

The program finds the intersection of two lists and then checks if the third list is a subset of the intersection.

Screenshot of the error:

Code:

list.h:

#ifndef __LIST_H_INCLUDED__
#define __LIST_H_INCLUDED__
#include <string>
#include <iostream>
#include <fstream>

struct node
{
    int value;
    node *next;
};

class list
{
    node* head;
public:
    list();
    ~list();
    void AddNodes(std::istream &input);
    void PrintList(std::ostream &output = std::cout);
    void AddOneNode(int AddVal);
    node* RetHead();
    list* Intersection(list* list2);
    bool IsPresent(int val);
    bool Subset(list subset);
 };

 #endif

      

        
        
        
      

    

list.cpp:

#include "stdafx.h"
#include "list.h"
#include <iostream>
#include <fstream>


list::list()
{
    head=NULL;
}

list::~list()
{

    node* current = head;
    while( current != 0 ) 
    {
        node* next = current->next;
        delete current;
        current = next;
    }
    head = 0;

}

void list::AddNodes(std::istream &input)
{
    int InVal;
    while(input>>InVal)
        AddOneNode(InVal);
}

void list::AddOneNode(int AddVal)
{
    node *NewNode= new node;
    NewNode->value=AddVal;
    NewNode->next=NULL;
    if(!head)
        head=NewNode;
    else
        {
            node *temp=head;
            while(temp->next)
                temp=temp->next;
            temp->next=NewNode;
        }
}

void list::PrintList(std::ostream &output)
{
    node *temp=head;
    while(temp)
    {
        output<<temp->value<<std::endl;
        temp=temp->next;

    }
}

list* list::Intersection(list *list2)
{
    list* result=new list;
    node* temp1=head;
    while(temp1)
    {
        if(list2->IsPresent(temp1->value))
            result->AddOneNode(temp1->value);
        temp1=temp1->next;

    }
    return result;
}

bool list::IsPresent(int val)
{
    node *temp=head;
    while(temp)
    {
        if(temp->value==val)
            return true;
        temp=temp->next;
    }
    return false;
}


bool list::Subset(list subset) // head=set
{
    bool flag;
    node* tempset=head;
    node* tempsub=subset.RetHead();
    while(tempset)
    {
        if (tempsub->value==tempset->value)
        {
            flag=true;
            break;
        }
        tempset=tempset->next;
    }
    if (!tempset)
        return false;
    while(tempsub)
    {
        tempsub=tempsub->next;
        if(!tempsub)
            return true;
        while(tempsub->value!=tempset->value&&tempset)
            tempset=tempset->next;
        if(!tempset)
            return false;
    }
    return flag;
}

node* list::RetHead()
{
    return head;
}

      

        
        
        
      

    

main.cpp:

#include "stdafx.h"
#include "list.h"
#include <Windows.h>
#include <fstream>

list Cross (list list1, list list2);
bool Subset (list set, list subset);

int main()
{
    setlocale (LC_ALL, "Russian");
    list l1,l2,l3;
    std::ifstream fl1 ("l1.txt");
    std::ifstream fl2 ("l2.txt");
    std::ifstream fl3 ("l3.txt");
    l1.AddNodes(fl1);
    std::cout<<"List 1:"<<std::endl;
    l1.PrintList();
    std::cout<<std::endl;
    l2.AddNodes(fl2);
    std::cout<<"List 2:"<<std::endl;
    l2.PrintList();
    std::cout<<std::endl;
    l3.AddNodes(fl3);
    std::cout<<"List 3:"<<std::endl;
    l3.PrintList();
    std::cout<<"Intersection of list 1 and list 2"<<std::endl;
    list *intersec=l1.Intersection(&l2);
    intersec->PrintList();
    std::cout<<std::endl;
    if(intersec->Subset(l3))
        std::cout<<"Third set is a subset of the intersection"<<std::endl;
    else
        std::cout<<"Third set is not a subset of the intersection"<<std::endl;
    system("pause");
    return 0;
}