debug help - swap 2 nodes of double link list

Question

Could you please help me debug this code to swap two node of double link list? I am not able to figure out what i am doing wrong :(

here is the code:

Answer 1

Suppose node1->next == node2 && node2->prev == node1. Now let's trace:

if(node1->next!=NULL)
{
   node1->next->prev=node2;
} 

Now node2->prev points to node2 itself!

if(node2->prev!=NULL)
{
    node2->prev->next=node1;
}

Now node2->next points to node1, which is ok for now.

Recall that node1->next still points to node2, and node2->next points to node1.

tmp=node1->next;  // == node2
node1->next=node2->next; // == node1 (!)
node2->next=tmp;  // == node2

So we have node1->next pointing to node1, and node2->next to node2. Clearly wrong.

Recall that node2->prev points to node2, although node1->prev is correct.

tmp=node1->prev; // correct
node1->prev=node2->prev; // == node2
node2->prev=tmp; // correct

So node1->prev points to node2, which is correct.

But node1->next and node2->next are still wrong!

---

How to solve this? It's not a one-liner to solve, as there are a couple special cases.

Maybe detect the special case I described and have separate code for it (and don't forget about that other special case).

Writing that code is left as an exercise to the reader ;)