Double linked list insert delete

#include <iostream>

using namespace std;

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

void add(int x)
{
    if(first == NULL)
    {
        first = new Node;
        first->data = x;
        first->previous = NULL;
        first->next = NULL;
        last = first;
    }
    else
    {
        Node *t = new Node;
        t->data = x;
        t->previous = last;
        t->next = NULL;
        last->next = t;
        last = t;
    }
}

void display(struct Node *p)
{
    while(p)
    {
        cout << p->data << " ";
        p = p->next;
    }
}
void displayReverse(struct Node*p)
{
    while(p)
    {
        cout << p->data << " ";
        p = p->previous;
    }
}

void insertAfter(int key, int val)
{
    Node *p = first;
    if(first->data == key)
    {
        Node *t = new Node;
        t->data = val;
        t->previous = first;
        t->next = first->next;
        first->next->previous = t;
        first->next = t;
    }
    else
    {
        while(p->data!=key)
        {
            p = p->next;
        }
        Node *t = new Node;
        t->data = val;
        t->next = p->next;
        t->previous = p;
        p->next = t;
    }

}

void deleteAfter(int key)
{


    Node *p = first;
    if(key == first->data)
    {
        Node *t = first;
        first = first->next;
        first->previous = NULL;
        delete t;

    }
    else
    {
        while(p->data !=key )
        {
            p = p->next;
        }
        Node *t = new Node;
        t = p->next;
        if(t->next == NULL)
        {
            p ->next = NULL;
        }
        else
        {
          p->next =p->next->next;
          p->next->previous = p;
        }
        delete t;
    }
}

void insertBefore(int key, int val)
{
    Node *p = first;
    if(p->data == key)
    {
        Node *t = new Node;
        t->next = first->next;
        t->previous = NULL;
        t->data = val;
        first = t;
    }
    else
    {
        while(p->data!=key)
        {
            p = p->next;
        }
        Node *t = new Node;
        t->data = val;
        t->next = p;
        t->previous = p->previous;
        p->previous->next = t;
        p->previous = t;
    }
}

int main()
{
    add(11);
    add(2);
    add(3);
    add(5);
    add(6);
    add(4);
    display(first);
    cout << endl;
    //insertAfter(2,10);
    //deleteAfter(6);
    insertBefore(3,9);
    display(first);

}