Find the same elements in C-struct - c++

Question

I have to write a function that will add elements to C-struct, but it can't add the same element. Example:
Input:
1 2 1 3
Output:
ADDED 1
ADDED 2
NOT ADD 1
ADD 3
Elements are taken from array, here's piece of code that uses the function I need to write:
int tab[] = {1,4,1,3,5};
Node* head = 0;
for (size_t i = 0, e = std::size(tab); i != e; ++i) {
bool b = add(head,tab[i]);
cout << tab[i] << (b ? " " : " NOT ")
<< "added" << endl;
}
C-struct Node looks like that:
struct Node {
int data;
Node* next;
};
Here's what I wrote, but it adds all elements from array. I can't change the loop, only add function:
bool add(Node*& head, int data){
Node *n = new Node;
n->data = data;
n->next = 0;
if(!head)
head = n;
else{
Node *tmp = head;
while(tmp->next)
tmp = tmp->next;
tmp->next = n;
}
};

Answer 1

currently you just add the element without looking if it is already present or not
The definition can be something like
bool add(Node*& head, int data){
if(!head) {
head = new Node;
n->data = data;
n->next = 0;
return true;
}
Node *tmp = head;
while (tmp->next) {
if (tmp->data == data)
return false;
tmp = tmp->next;
}
if (tmp->data == data)
return false;
tmp->next = new Node;
tmp->next->data = data;
tmp->next->next = 0;
return true;
}
I encourage you to add a constructor to not have to set the data and next fields each time after you create a new instance
Exemple
Node::Node(int d) : next(0), data(d) {
}
// add should be a static method of Node, to be able to access next and data while they are private
bool add(Node*& head, int data){
if(!head) {
head = new Node(data);
return true;
}
Node *tmp = head;
while (tmp->next) {
if (tmp->data == data)
return false;
tmp = tmp->next;
}
if (tmp->data == data)
return false;
tmp->next = new Node(data);
return true;
}

Answer 2

Here's my attempt. Look for the existing data first then add if not present (no change there from existing code)
bool add(Node*& head, int data) {
Node *tmp = head;
while (tmp) {
if (tmp->data == data)
return false; // data already present
tmp = tmp->next;
}
Node *n = new Node;
n->data = data;
n->next = 0;
if (!head) {
head = n;
}
else {
Node *tmp = head;
while(tmp->next)
tmp = tmp->next;
tmp->next = n;
}
return true; // data added
}

Answer 3

So I did something like that and it works with the data I have. I suppose it works in general
bool add(Node*& head, int data){
Node *n = new Node;
n->data = data;
n->next = 0;
if(!head)
head = n;
else{
Node *tmp = head;
while(tmp->next){
if(tmp->data == data)
return false;
else
tmp = tmp->next;
}
tmp->next = n;
}
};