doubly linked list

Hello

How I can change the list, so that it can not enter duplicate value ?

and this list is correct ?

Thanks.

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/* Double linked list */

#include <iostream>

using namespace std;

class Nodo {
   private:
    int value;
    Nodo* prev;
    Nodo* next;
   public:
    Nodo();
    Nodo(int val, Nodo* pr, Nodo* ne);
    friend class Lista;
};

     Nodo::Nodo() {
        value = 0;
        prev = NULL;
        next = NULL;
    }

    Nodo::Nodo(int val, Nodo* pr, Nodo* ne) {
        value = val;
        prev = pr;
        next = ne;
    }

class Lista {
   private:
    Nodo* head;
    Nodo* tail;
   public:
    Lista();
    ~Lista();
    void inserisciLista(int val);
    void cancLista(int val);
    void stampaLista();
    void stampaRevList();
};

     Lista::Lista() {
        head = NULL;
        tail = NULL;
    }

    Lista::~Lista() {
        Nodo *ptr, *temp;
        ptr = head;
        // go thru the list and delete all nodes
        while (ptr != NULL) {
            temp = ptr->next;
            delete(ptr);
            ptr = temp;
        }
      }

      void Lista::inserisciLista(int val) {
        // sorted inserisciLista. from smallest to largest number
        Nodo *ptr;
        ptr = head;

        if (ptr == NULL) { // 1st: inserisciListaing first element
            head = new Nodo(val, NULL, NULL);
            tail = head;
            return;
        }

        // traverse to the right place
        while (ptr->next != NULL && ptr->value <= val) {
            ptr = ptr->next;
        }

        if (ptr == head && ptr->value > val) { // 2nd: if inserisciListaing
                                                // to head
            head = new Nodo(val, NULL, head);
            head->next->prev = head;
        }
        else if (ptr == tail && ptr->value <= val) { // 3rd: inserisciListaing
                                                      // to end
            tail = new Nodo(val, tail, NULL);
            tail->prev->next = tail;
        }
        else { // 4th: inserisciListaing to middle
            ptr->prev = new Nodo(val, ptr->prev, ptr);
            ptr->prev->prev->next = ptr->prev;
        }
    }

     void Lista::cancLista(int val) {
        Nodo *ptr;
        ptr = head;
        // this function cancListas the first node found of this value

        while (ptr != NULL && ptr->value != val) {
            ptr = ptr->next;
        }

        if (ptr != NULL) {
            // if ptr went to NULL, it went thru the list and didnt find value
            // in that case, ignore. only do the cancLista
             // when theres something to.
            if (ptr == head) {
                // if removing head node
                head = head->next;
                head->prev = NULL;
                delete(ptr);
            } else if (ptr == tail) {
                // if removing tail node
                tail = tail->prev;
                tail->next = NULL;
                delete(ptr);
            } else {
                // if removing in the middle
                ptr->prev->next = ptr->next;
                ptr->next->prev = ptr->prev;
                delete(ptr);
            }
        }
    }

    void Lista::stampaLista() {
        Nodo *ptr;
        ptr = head;
        while (ptr != NULL) {
            cout<<ptr->value<<" ";
            ptr = ptr->next;
        }
        cout<<endl;
    }

    void Lista::stampaRevList() {
        Nodo *ptr;
        ptr = tail;
        while (ptr != NULL) {
            cout<<ptr->value<<" ";
            ptr = ptr->prev;
        }
        cout<<endl;
    }

int main() {
    Lista mylist;
    int input;
    int scelta;
    char select;

     do{
        cout << "-------- MENU --------" << endl;
        cout << "1. Inserisci valore nella lista." << endl;
        cout << "2. Stampa la lista." << endl;
        cout << "3. Rimuovi valore dalla lista." << endl;
        cout << "4. Rovescia la pila." << endl;
        cout << "5. Stampa il numero degli elementi." << endl;
        cout << "8. Esci." << endl;
        cout << ": ";
        cin >> scelta;
        switch(scelta){
           case 1:
                cout << "Inserisci un valore nella lista: ";
                cin >> input;
                mylist.inserisciLista(input);
                cout << "Vuoi inserire un altro valore ? S/N: ";
                cin >> select;
                while(select != 'n' && select != 'N'){
                   cout << "Inserisci un valore nella lista: ";
                   cin >> input;
                   mylist.inserisciLista(input);
                   cout << "Vuoi inserire un altro valore ? S/N: ";
                   cin >> select;
                }
           break;

           case 2:
                cout << "Lista: " << endl;
                mylist.stampaLista();
           break;

           case 3:
                cout << "Inserisci valore da rimuovere: ";
                cin >> input;
                mylist.cancLista(input);
                cout << "Vuoi rimuovere un altro valore ? S/N: ";
                cin >> select;
                while(select != 'n' && select != 'N'){
                   cout << "Inserisci valore da rimuovere: ";
                   cin >> input;
                   mylist.cancLista(input);
                   cout << "Vuoi rimuovere un altro valore ? S/N: ";
                   cin >> select;
                }
           break;

           case 8:

           break;

           default:
                cout << "!*!*!*!* SCELTa NON VALIDA *!*!*!*!" << endl <<
endl;
        }
     }
     while(scelta != 8);

     system("PAUSE");
    return 0;
}

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