add one ll c++

#include <iostream>
#include <stack>

using namespace std;




class node{

protected:
    int element;
    node* link;

public:
    //constructor that accepts only element
    node(int element) {
        this->element = element;
        this->link = NULL;
    }

    //constructor that accepts both link and element
    node(int element, node* link){
        this->element = element;
        this->link = link;
    }

    //method to update the element
    void updateData(int element){
        this->element = element;
    }

    //method to update or setup link
    void updateLink(node* link){
        this->link = link;
    }

    //method to get the element from the node
    int getElement(){
        return this->element;
    }

    //method to get the next node
    node* getNextNode(){
        return this->link;
    }
};

class Linkedlist {
public:
    node *head;

    //constructor for the Linked List class
    Linkedlist() {
        head = NULL;
    }

    //returns head node
    node *getHead() {
        return this->head;
    }

    // method to add a node at the end
    void insert(int element) {
        node *tempNode = new node(element);
        node *p = head;
        if (head == NULL) {
            head = tempNode;
            return;
        }
        else {
            while (p->getNextNode() != NULL) {
                p = p->getNextNode();
            }
            p->updateLink(tempNode);
            return;
        }
    }

    //method to display all the elements of the Linked List
    void display() {
        cout << "\n";
        node *tempNode = head;
        while (tempNode != NULL) {
            if (tempNode->getNextNode() != NULL)
                cout << tempNode->getElement() << " --> ";
            else
                cout << tempNode->getElement();

            tempNode = tempNode->getNextNode();
        }
        return;
    }

    void addOne(){
        this->reverse(this->head);
        addingOneUtil();
        this->reverse(this->head);
    }

    private:
        // adding one to linked list
        void addingOneUtil(){
            int carry = 1;
            int sum = 0;

            node* start = this->head;
            while(start != NULL && carry != 0){
                sum = start->getElement() + carry;
                start->updateData(sum % 10);
                carry = sum / 10;
                start = start->getNextNode();
            }

            if(carry != 0){
                this->insert(carry);
            }
        }

    // function to reverse linked list
    void reverse(node* head){
        if(head == NULL){
            return;
        }

        node* next = NULL;
        node* prev = NULL;
        node* current = head;
        while(current != NULL){
            next = current->getNextNode();
            current->updateLink(prev);
            prev = current;
            current = next;
        }
        this->head = prev;
    }
};

int main() {
    cout<<"Program to add one to a number represented by Linked List";
    Linkedlist l1;
    l1.insert(9);
    l1.insert(9);
    l1.insert(9);
    l1.insert(9);
    l1.insert(9);
    l1.insert(9);
    l1.insert(9);
    l1.insert(9);
    l1.display();
    cout<<"\nNumber after adding one is : ";
    l1.addOne();
    l1.display();
    return 0;
}