debug help - swap 2 nodes of double link list

问题

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:

dll* swap_node(dll *head , dll *node1 , dll *node2) {
   dll *tmp;
   int flag=0;

   if(node1->prev!=NULL) {
       node1->prev->next=node2;
   } else {
       flag=1;
   }
   if(node1->next!=NULL) {
       node1->next->prev=node2;
   }

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

   tmp=node1->next;
   node1->next=node2->next;
   node2->next=tmp;

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

   if(flag==1) {
       head=node2;
   }
   return head;
}

Thanks in advance

回答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 ;)

回答2:

Your logic will not work,

If node2 is the first element in the doubly linked list
If node1 and node2 are adjacent.

Please fine the updated logic given below.


dll* swap_node(dll *head , dll *node1 , dll *node2) 
{ 
    dll* previous_to_node1 = NULL;
    dll* next_to_node1 = NULL;
    dll* previous_to_node2 = NULL;
    dll* next_to_node2 = NULL;

    if ((node1 == NULL) || (node2 == NULL))
         return 0;

    previous_to_node1 = node1->previous;
    next_to_node1 = node1->next;
    previous_to_node2 = node2->previous;
    next_to_node2 = node2->next;

    if (previous_to_node1 != NULL) previous_to_node1->next = node2;
    if (next_to_node1 != NULL) next_to_node1->previous = node2;
    if (pevious_to_node2 != NULL) previous_to_node2->next = node1;
    if (next_to_node2 != NULL) next_to_node2->previous = node1;

    node1->next=next_to_node2;    
    node1->previous=previous_to_node2;
    node2->next=next_to_node1;
    node2->previous=previous_to_node1;

    if (previous_to_node1 == NULL) 
    {
        return node2;
    }
    else if(previous_to_node2 == NULL)
    {
        return node1;
    }

    return head;
}

来源：https://stackoverflow.com/questions/8306267/debug-help-swap-2-nodes-of-double-link-list

标签

data-structures

linked-list