Stack basic push and pop

#include <iostream>
using namespace std;

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

class stack{
    node *top = new node;
public:
    stack(){
        top = NULL;
    }
    void push(int v){
        node *nn = new node;
        nn -> data = v;
        nn -> next = top;
        top = nn;
    }

    int pop(){
        if(top == NULL){
            cout << "Stack is empty " << endl;
            return 0;
        }
        else {
            node *t = top;
            int val = top-> data;
            top = top -> next;
            free(t);
            return val;
        }
    }

    void display(){
        node *t=top;
        while(t!=NULL){
            cout << t->data <<" -> ";
            t=t->next;
        }
        cout<<"NULL\n";
    }
};

int main(){
    stack s;
    int ch = 10;
    while(ch != 0){
        cout << "Enter your choice" << endl;
        cout << "1: Push an element " << endl;
        cout << "2: Pop an element " << endl;
        cout << "0: Exit " << endl;
        cin >> ch;
        switch(ch){
        case 1:
            int value;
            cout << "Enter value to push" << endl;
            cin >> value;
            s.push(value);
            s.display();
            break;
        case 2:
            cout << "Popped element: " << s.pop() << endl;
            s.display();
            break;
        }
    }
    return 0;
}