Users dissapear when I sort a doubly-linked list using quicksort

Question

I have a doubly-linked list and I want to sort the users by their ELO.

The problem is that when the function swap() is called, some users dissapear

This is what I get in my console: https://imgur.com/a/pJSPSnW

As you can see, the first swap (between players 1 and 2) is done correctly. But, when it swaps players 3 and 4, user1 dissapears.

I think that the problem is inside the function swap(), but I can't figure out where exactly

Code: User/Node declaration

struct user {
public:
    string username;
    float ELO = NULL; //Score of the user       
    user* next = nullptr;
    user* previous = nullptr;
};

QuickSortRecursive function

 void ELOManager::_quickSortRecursive(user* low, user* high) {
    if (high != nullptr && low != high && low != nullptr) {
        user* p = partition(low, high);
        _quickSortRecursive(low, p->previous);
        _quickSortRecursive(p->next, high);
    }
}

QuickSort function

void ELOManager::quickSort(){
    _quickSortRecursive(first, last);
}

Partition function

    user* ELOManager::partition(user* low, user* high) {
    float pivot = high->ELO;

    user* i = low->previous;

    for (user* j = low; j != high && j!=nullptr; j = j->next) {
        if (j->ELO <= pivot){
            i = (i == NULL) ? low : i->next;

            swap(i, j);
        }
    }
    //i = (i == NULL) ? low : i->next;
    swap(i->next, high);
    cout << endl << "Loop finished -----------------------" << endl;
    printUsers();
    return i;
}

Swap function

    void ELOManager::swap(user* A, user* B) {
    user* tmp = new user();
    user* swapperVector[4];

    cout << endl << "swap1[" << A->username << "]" << endl;
    cout << "swap2[" << B->username << "]" << endl;

    if (A == B) {
        cout << "Same Users: Continue" << endl;
        return;
    }   

    swapperVector[0] = A->previous;
    swapperVector[1] = B->previous;
    swapperVector[2] = A->next;
    swapperVector[3] = B->next; 

    if (areTheyNeighbours(A, B)) {
        A->previous->next = B;
        A->next->previous = B;
        B->previous->next = A;
        B->next->previous = A;


        A->previous = B;
        B->previous = swapperVector[1];
        A->next = swapperVector[3];
        B->next = A;    
        cout << endl << "Option 1" << endl;
    }
    else {
        A->previous = swapperVector[1];
        B->previous = swapperVector[0];
        A->next = swapperVector[3];
        B->next = swapperVector[2];

        A->previous->next = B;
        A->next->previous = B;
        B->previous->next = A;
        B->next->previous = A;
        cout << endl << "Option 2" << endl;
    }

    cout <<"Print list after swap" << endl << "-----" << endl;
    printUsers();
}

Feel free to get into my project's github https://github.com/pablogalve/ELO-System

I would appreciate your help :)

Answer 1

In your if (areTheyNeighbors(A, B)) block, your linked list pointers end up looking like this: Notice that:

• Both of B's pointers are pointed at A
• Both A and A->next think B is their previous list element

This causes multiple problems in list traversal, and might be what causes the first element in the list to be lost.

If A and B are neighbors, this should swap them properly.

A->previous = B;
B->previous = swapperVector[0];
A->next = swapperVector[3];
B->next = A;

Answer 2

Amanda's answer shows one way to deal with adjacent nodes, but there doesn't need to be a special case to handle adjacent versus non-adjacent nodes, if the swapping is done first by swapping what is pointing to the two target nodes, then swapping the pointers within the two target nodes.

    AP = A->previous;
    BP = B->previous;
    AN = A->next;
    BN = B->next;
    std::swap(AN->previous, BN->previous);
    std::swap(AP->next, BP->next);
    std::swap(A->previous, B->previous);
    std::swap(A->next, B->next);