a little bug in : 14_LinkedLists/02_OOLinkedListWithIterator.cpp

[Baxayesh]

for this function:

List::Node *List::make_copy(List::Node *p) {    return p ? new Node(p->data, p->prev, make_copy(p->next)) : NULL;}

consider we have a linked list like this in heap:
`1
0. address: prev | value | next

1. X001: null | 10 | X002
2. X002: X001 | 20 | X003
3. X003: X002 | 30 | null

by calling function:

• 1.stack frame : P = X001 return ?
• • p != null --> calling make_copy again :
• 2.stack frame : P = X002 return ?
• • p != null --> calling make_copy again :
• 3.stack frame : P = X003 return ?
• • p != null --> calling make_copy again :
• 4.stack frame : P = null return ?
• • p == null --> return null

so in third stack frame : p = X003

• return : new Node(p->data, p->prev, (return of fourth stack frame [null]) )

•                     -->consider  new memory block is : X103 : X002 | 30 | null
  

  in second stack frame : p = X002

• return : new Node(p->data, p->prev, (return of third stack frame [X103]) )

•                          ->consider  new memory block is : X102 : X001 | 20 | X103
  

  in first stack frame : p = X002

• return : new Node(p->data, p->prev, (return of second stack frame [X102]) )

•                          ->consider  new memory block is : X101 : null | 10 | X102 
  

now heap is:

• -first list:

1. X001: null | 10 | X002
2. X002: X001 | 20 | X003
3. X003: X002 | 30 | null

• -second list:

4. X101: null | 10 | X102
5. X102: X001 | 20 | X103
6. X103: X002 | 30 | null

and as you see prev pointers of any node in second list are pointing to first list elements!!

I guess this can solve problem:

List::Node *List::make_copy(List::Node *p) {
    if( p == nullptr )
        return nullptr;

    Node* tail = make_copy(p->next);
    Node* head = new Node(p->data, nullptr, tail);

    if(tail != nullptr)
        tail->prev = head;
        
    return head;
}