for (node* c = head; c != NULL; c = c->y)
    {
        for (node* prev = head, *current = head->y, *next = head->y->y; next != NULL; prev = prev->y, current = current->y, next = next->y)
        {
            if (next->x < current->x)
            {
                node* x = prev->y;
                prev->y = current->y;
                node* y = next->y;
                next->y = x;
                current->y = y;
            }
        }
    }
//error:Exception thrown: read access violation.
prev was nullptr.