implementing doubly linked list in C++

Question

I have trouble implementing a doubly linked list, especially in a function that swaps adjacent elements.

Here is my original code:

#include <iostream>
using namespace std;

struct node {
    int data; 
    node *prev;
    node *next;
};
node *head = NULL;

node* getNewNode (int i){
    node *newNode = new node; //allocate a dynamic memory in the heap
    newNode->data = i;
    newNode->prev = NULL;
    newNode->next = NULL;
    return newNode;
}

void InsertAtHead(int i){
    node *newNode = getNewNode(i);
    if (head == NULL){   //addressing inserting the first node in the list.
        head =  newNode;
        return;
    }
    
    newNode->next = head;
    head->prev = newNode;
    head = newNode;    
}

void swapAdjacent() {
    node *temp = head;
    while (temp != NULL ) {
        node *temp1 = temp;
        node *temp2 = temp->next;
        temp1->next = temp2->next;
        temp2->prev = temp1->prev;
        if (temp1->prev != NULL) {
            temp1->prev->next = temp2;
        }
        temp1->prev = temp2;
        temp2->next = temp1;
       
        if (temp == head) {
            head = temp2;
        }
        temp = temp->next->next;
    }
}

void display (){
    node *temp = head;
    while (temp != NULL){
        cout  <<temp->data << '\t';
        temp = temp->next;    
    }
    cout << '\n';
}

int main (){
    InsertAtHead(8); 
    InsertAtHead(4);
    InsertAtHead(12);
    InsertAtHead(3);
    InsertAtHead(-8);
    InsertAtHead(7);
    cout << "The original list is: " << endl;
    display();
    swapAdjacent();
    cout << "The list after swapping the adjacent elements is: " << endl;
    display();
       
    return 0;
}

The original list is:

7       -8      3       12      4       8

The output I got after calling swapAdjacent() is:

-8      7       3       4       12      8

While I am looking for:

-8     7       12      3        8       4

I tried visualizing this with pen and paper, did several attempts to look it up, and I even used ChatGPT, but to no avail. I always don't get the output I am looking for.

My question is that I want the swapAdjacent() function to produce the desired output and not the output it is currently producing.

Answer 1

Thanks everyone for your feedback. I figure out what was wrong with my code which basically I forget to update the pointer of the next node (prev variable).Also, I was going forward one node extra. Here is my updated code. I welcome all of your feedback.

#include <iostream>
using namespace std;

struct node {
int data; 
node *prev;
node *next;
};

node *head = NULL; 

node* getNewNode (int i){
node *newNode = new node;  //allocate a dynamic memory in the heap
newNode->data = i;
newNode->prev = NULL;
newNode->next = NULL;
return newNode;
}

void InsertAtHead(int i){
node *newNode = getNewNode(i);
if (head == NULL){    //addressing inserting the first node in the list.
    head =  newNode;
    return;
}
newNode->next = head;
head->prev = newNode;
head = newNode;    
}

void swapAdjacent() {
node *temp = head;
while (temp != NULL && temp->next!=NULL ) {
    node *temp1 = temp;
    node *temp2 = temp->next;
     if(temp2->next != NULL){
        temp2->next->prev = temp1;
    }
    temp1->next = temp2->next;
    temp2->prev = temp1->prev;
    if (temp1->prev != NULL) {
        temp1->prev->next = temp2;
    }
    temp1->prev = temp2;
    temp2->next = temp1;
   
    if (temp == head) {
        head = temp2;
    }
   
    temp = temp->next;
}
}

void display (){
node *temp = head;
while (temp != NULL){
    cout  <<temp->data << '\t';
    temp = temp->next;    
}
cout << '\n';
}

int main (){
InsertAtHead(8); 
InsertAtHead(4);
InsertAtHead(12);
InsertAtHead(3);
InsertAtHead(-8);
InsertAtHead(7);
cout << "The original list is: " << endl;
display();
swapAdjacent();
cout << "The list after swapping the adjacent elements is: " << endl;
display();
   
return 0;
}