It is a program that combines two linked lists and displays the list.I've done what I want to do, but it doesn't end normally.

#include<iostream>
# include "TextClass.h"

using namespace std;

int main() {
    const int APPEND1 = 6;
    const int APPEND2 = 7;
    int counter = 0;
    char appendVals [APPEND1+APPEND2] = {'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm'};

    TextClass first;
    for (inti=0;i<APPEND1;i++)
    {
        first.addTail (appendVals [counter]);
        counter++;
    }
    TextClass second;
    for (inti=0;i<APPEND2;i++)
    {
        second.addTail(appendVals[counter]);
        counter++;
    }

    std::cout<<"First list is"<<first.displayList()<<std::endl;
    std::cout<<"Second list is"<<second.displayList()<<std::endl;

    first.append(second);
    std::cout<<"First should now be a bc de fghijklm"<<std::endl;
    std::cout<<"and it actually is"<<first.displayList()<<std::endl;

    std::cout<<"Done applying a list\n"<<std::endl;


    return 0;
}

#include "TextClass.h"
# include <sstream>
// constructor
Link:: Link (charletter, Link*next, Link*prev)
{
    value = letter;
    this ->next=next;
    this ->prev=prev;
}

// destructor
Link::~Link(){}

// return the value in the link
char Link::getValue() {return value;}

// set a new next address
void Link::setNext(Link*next) {this->next=next;}

// set a new previous address
void Link::setPrev(Link*prev) {this->prev=prev;}

// return the next address
Link*Link::getNext() {return this ->next;}

// return the previous address
Link*Link::getPrev() {return this ->prev;}


// constructor
TextClass:: TextClass()
{
    // head and tail are set to nullptr
    head = nullptr;
    tail = nullptr;
}

// destructor
TextClass::~TextClass()
{
    // call removeHead until head = nullptr;
    while(head!=nullptr)
    {
        removeHead();
    }
}


// add value at tail
voidTextClass::addTail(charletter)
{
    // if head == nullptr
    if(tail==nullptr)
    {
        // head and tail = new link
        head=tail=newLink(letter);
    }
        // else
    else
    {
        // create new link and insert it at tail
        Link*temp = new Link (letter, nullptr, tail);
        tail->setNext(temp);
        // change tail
        tail = temp;
    }
}


// return the contents of list
US>string TextClass::displayList()
{

    // create a variable that stores contents for string and stringstream
    string output;
    std::stringstreams;
    // create a link that walks down the list and it starts from head
    Link* temp=head;
    // while the link!= nullptr
    while(temp!=nullptr)
    {
        // copy the value in the link to stringstream
        ss<<temp->getValue()<
        // move the link forward
        temp=temp->getNext();
    }

    // copy the value in stringstream to string
    output =ss.str();

    // return the string
    return output;
}

// connect two lists
voidTextClass::append(TextClass otherList)
{
    // create a link that stores the head of otherList
    Link*temp=otherList.head;

    // connect tworinks
    // tail next should point to head of otherLink
    while(temp!=nullptr)
    {
        addTail(temp->getValue());
        temp=temp->getNext();
    }

}

// remove head
voidTextClass::removeHead()
{
    // save the link at head to delete later
    Link* temp=head;
    // update head
    head=head->getNext();
    // if head is null after update
    if(head==nullptr)
    {
        //tail=nullptr
        tail = nullptr;
    }
        // else update preview of new link
    else
    {
        head->setPrev(nullptr);
    }
    // now delete the old link
    delete temp;
}

#include<iostream>


using std::string;

classLink {
private:
    char value; // stores chat type value
    Link * next; // stores the address of next link
    Link*prev;//stores the address of previous link

public:
    Link(charletter, Link*next=nullptr, Link*prev=nullptr); // constructor
    ~Link();//destructor
    chargeValue(); // return the value in the link
    void setNext(Link*next); // set a new next address
    void setPrev (Link*prev); // set a new previous address
    Link*getNext(); // return the next address
    Link* getPrev(); // return the previous address
};


classTextClass{
private:
    Link*head;//track the head link of queue
    Link*tail;//track the tail link of queue
public:
    TextClass(); // constructor
    ~TextClass();//destructor

    void addTail (charletter); // add value at tail
    string displayList(); // return the contents of list
    void append (TextClass otherList); // connect two lists
    void removeHead(); // remove head
};