Union of two linked lists [closed] - c++

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I am trying to develop a friend function for the union of two link lists. Compiler keeps giving me an error saying that Node was not declared in this scope, as well as p,q not declared in this scope. So I added List::Node, but it is not performing the union still
const List getunion(const List&a, const List&b) {
List::Node * p=a.list;
List::Node* q=b.list;
List result;
while (q!=NULL && p!=NULL) {
if(q->value==p->value) {
result.insert(q->value);
q=q->next;
p=p->next;
}
q=q->next;
p=p->next;
}
return result;
}
void insert(int x) {
Node* tmp=new Node;
tmp->value=x;
if(list==NULL || list->value >x){
tmp->next=list;
list=tmp;
}
else {
Node *curr=list;
while (curr->next !=NULL && curr->next->value < x) {
curr=curr->next;
}
tmp->next=curr->next;
curr->next=tmp;
}
Note my insert function adds Nodes in ascending order, and I have tried it out and it does work on test cases, it's when i implement the union function that things go astray...

Answer 1

I take it that you fixed the compile error and now you are wondering why it doesn't work.
You're not merging these two lists correctly. The issues are:
You only insert an item if it's in both lists - that's intersection, not union.
You are advancing both pointers simultaneously.
You need to merge one or the other:
while ( q && p ) {
if( q->value < p->value ) {
result.insert(q->value);
q = q->next;
} else if( q->value > p->value ) {
result.insert(p->value);
p = p->next;
} else {
result.insert(q->value);
q = q->next;
p = p->next;
}
}
Now, once either of those list pointers reaches the end, you still need to add the remaining elements from the other. This will do the trick:
for(; q; q = q->next) result.insert(q->value);
for(; p; p = p->next) result.insert(p->value);
Make sure, also, that you have defined a copy constructor for List. It looks like you're doing your own memory allocation in there, so you must follow the Rule of Three.

Answer 2

Firstly, I am not sure what you code is intended to be doing.
It seems you expect the input to be 2 lists, which you step thru in parallel, constructing a third list if the corresponding values are matching.
This is more like an intersection than a union, tho its harsher than that, since it cares about position.
The insert seems to be a sorted insert.
Anyway, this part is certainly wrong:
if(q->value==p->value) {
result.insert(q->value);
q=q->next;
p=p->next;
}
Remove these q=q->next and p=p->next.
These are causing a double step, since its done inside the if statement, and outside.
Double stepping is bad, since it can take you past the end of a list, causing crashes.