交换双链表

Question

Okay.

So I've been trying to implement a doubly-linked list with a swap function.

I've found many sites online that explain how to do this - and in almost all of them (including a few from Stack Overflow), an argument begins in the comments about whether or not the implementation is correct. I have tried several of them to no avail. In other words, I can't find a single definitive version of this algorithm.

I have tried at least 4-5 different algorithms from those I found online, as well as a few I tried to sort out myself, but I keep getting issues with the elements being sorted incorrectly. I am completely at my wits end.

Here is the most recent version I tried, which does not work:

template <typename T>
void PriorityQueue<T>::swap(Node * n1, Node * n2){
    if(n1 == n2)
        return;
    if(n1 == 0 || n2 == 0){
        std::cout << "\nPRIORITYQUEUE ERROR: Trying to swap with non-existent node\n";
        return;
    }

    Node * temp = new Node;

    temp->prev = n1->prev;
    temp->next = n1->next;

    n1->prev = n2->prev;
    n1->next = n2->next;

    n2->prev = temp->prev;
    n2->next = temp->next;

    if(n1->next != 0)
        n1->next->prev = n2;
    if(n1->prev != 0)
        n1->prev->next = n2;
    if(n2->next != 0)
        n2->next->prev = n1;
    if(n2->prev != 0)
        n2->prev->next = n1;

    delete temp;

}

I am beyond frustrated. Would somebody please, please, please help me out by showing me a complete, working algorithm. Not linking to a page with a broken algorithm and comments on how to fix it (which I have tried a few times now), but just the algorithm. It doesn't even need to be code, just an algorithm. Please. I am so tired of trying to fix this and so behind schedule on this project and I will be so eternally grateful if someone out there can definitely help me solve this once and for all.

Answer 1

Your problem in last lines. I corrected them:

if(n1->next != 0)
    n1->next->prev = n1; // n2;
if(n1->prev != 0)
    n1->prev->next = n1; // n2;
if(n2->next != 0)
    n2->next->prev = n2; // n1;
if(n2->prev != 0)
    n2->prev->next = n2; // n1;

In the beginning of the function you have two nodes n1 and n2 with pointers to other elements n1p , n1n , n2p , n2n .

<-| p |  <-| p |  <-| p |
  |n1p|    | n1|    |n1n|
  | n |->  | n |->  | n |->

<-| p |  <-| p |  <-| p |
  |n2p|    | n2|    |n2n|
  | n |->  | n |->  | n |-> 

In the first part with temp variable you swap pointers of n1 and n2 nodes. So you come to situation like this.

<-| p |  <-| p |  <-| p |
  |n1p|    | n2|    |n1n|
  | n |->  | n |->  | n |->

<-| p |  <-| p |  <-| p |
  |n2p|    | n1|    |n2n|
  | n |->  | n |->  | n |->

And you problem was that you incorrectly change pointers for n1p , n1n , n2p and n2n . Take a look in example with sinplified class Node and corrcted swap function.