Linked list duplicates remove with set

// Example program
#include <iostream>
#include <string>
#include <set>

using namespace std;

struct Node
{
    int data;
    struct Node* next;
} *first = NULL;

void add(int data)
{
        struct Node *p;

    if(first == NULL)
    {
        first = new  Node;
        first->data = data;
        first->next = NULL;

    }

    else
    {
        p = first;
        while(p->next !=NULL)
        {
            p = p->next;
        }
         struct Node *t = new struct Node;
    t->data = data;
    p->next = t;

    }


}

void print(struct Node *p)
{
    if(p)
    {
        cout << p->data << endl;
        print(p->next);
    }

}

void removeDuplicates(struct Node *p)
{
    set <int> s1;
    struct Node *prev = NULL;
    while(p)
    {

        if(s1.count(p->data) == 1)
        {
            prev->next = p->next;
        }
        else
        {
            s1.insert(p->data);

        }
         prev = p;
        p = p->next;

    }


}


int main()
{
  add(10);
  add(10);
  add(20);
  add(20);
  removeDuplicates(first);
  print(first);
}