XOR Linked List – Reversal of a List

Given a XOR linked list, the task is to reverse the XOR linked list.

Examples:

Input: 4 <–> 7 <–> 9 <–> 7
Output: 7 <–> 9 <–> 7 <–> 4
Explanation:
Reversing the linked list modifies the XOR linked list to 7 <–> 9 <–> 7 <–> 4.

Input: 2 <-> 5 <-> 7 <-> 4
Output: 4 <-> 7 <-> 5 <-> 2
Explanation:
Reversing the linked list modifies the XOR linked list to 4 <-> 7 <-> 5 <-> 2.

Approach: XOR linked listconsists of a single pointer, which is the only pointer needed to traverse the XOR linked list in both directions. Therefore, the idea to solve this problem is only by making the last node of the XOR linked list its Head Node. Follow the steps below to solve the problem:

1. Initialize a pointer variable, say curr, to pointto the current node being traversed.
2. Store the current head pointer in the curr variable.
3. If curr is equal to NULL, then return NULL.
4. Otherwise, traverse upto the last node and make it the head of the XOR linked list.

Below is the implementation of the above approach:

C++

// C++ program for the above approach
#include<bits/stdc++.h>
using namespace std;

// Structure of a node
// in XOR linked list
struct Node
{

    // Stores data value
    // of a node
    int data;

    // Stores XOR of previous
    // pointer and next pointer
    Node* nxp;
};

// Function to find the XOR of two nodes
Node* XOR(Node* a, Node* b)
{
    return (Node*)((uintptr_t)(a) ^ (uintptr_t)(b));
}

// Function to insert a node with
// given value at beginning position
Node* insert(Node** head, int value)
{

    // If XOR linked list is empty
    if (*head == NULL)
    {

        // Initialize a new Node
        Node* node
            = new Node();

        // Stores data value in
        // the node
        node->data = value;

        // Stores XOR of previous
        // and next pointer
        node->nxp = XOR(NULL, NULL);

        // Update pointer of head node
        *head = node;
    }

    // If the XOR linked list
    // is not empty
    else
    {

        // Stores the address
        // of current node
        Node* curr = *head;

        // Stores the address
        // of previous node
        Node* prev = NULL;

        // Initialize a new Node
        Node* node
            = new Node();

        // Update curr node address
        curr->nxp = XOR(node, XOR(NULL, curr->nxp));

        // Update new node address
        node->nxp = XOR(NULL, curr);

        // Update head
        *head = node;

        // Update data value of
        // current node
        node->data = value;
    }
    return *head;
}

// Function to print elements of
// the XOR Linked List
void printList(Node** head)
{

    // Stores XOR pointer
    // in current node
    Node* curr = *head;

    // Stores XOR pointer of
    // in previous Node
    Node* prev = NULL;

    // Stores XOR pointer of
    // in next node
    Node* next;

    // Traverse XOR linked list
    while (curr != NULL)
    {

        // Print current node
        cout<<curr->data<<" ";

        // Forward traversal
        next = XOR(prev, curr->nxp);

        // Update prev
        prev = curr;

        // Update curr
        curr = next;
    }
    cout << endl;
}

// Function to reverse the XOR linked list
Node* reverse(Node** head)
{

    // Stores XOR pointer
    // in current node
    Node* curr = *head;
    if (curr == NULL)
        return NULL;
    else
    {

        // Stores XOR pointer of
        // in previous Node
        Node* prev = NULL;

        // Stores XOR pointer of
        // in next node
        Node* next;
        while (XOR(prev, curr->nxp) != NULL)
        {

            // Forward traversal
            next = XOR(prev, curr->nxp);

            // Update prev
            prev = curr;

            // Update curr
            curr = next;
        }

        // Update the head pointer
        *head = curr;
        return *head;
    }
}

// Driver Code
int main()
{
    /* Create following XOR Linked List
    head-->40<-->30<-->20<-->10 */
    Node* head = NULL;
    insert(&head, 10);
    insert(&head, 20);
    insert(&head, 30);
    insert(&head, 40);

    /* Reverse the XOR Linked List to give
    head-->10<-->20<-->30<-->40 */
    cout << "XOR linked list: ";
    printList(&head);
    reverse(&head);
    cout << "Reversed XOR linked list: ";
    printList(&head);

    return 0;
}

// This code is contributed by rutvik_56.

C

// C program for the above approach

#include <inttypes.h>
#include <stdio.h>
#include <stdlib.h>

// Structure of a node
// in XOR linked list
struct Node {

    // Stores data value
    // of a node
    int data;

    // Stores XOR of previous
    // pointer and next pointer
    struct Node* nxp;
};

// Function to find the XOR of two nodes
struct Node* XOR(struct Node* a, struct Node* b)
{
    return (struct Node*)((uintptr_t)(a) ^ (uintptr_t)(b));
}

// Function to insert a node with
// given value at beginning position
struct Node* insert(struct Node** head, int value)
{

    // If XOR linked list is empty
    if (*head == NULL) {

        // Initialize a new Node
        struct Node* node
            = (struct Node*)malloc(sizeof(struct Node));

        // Stores data value in
        // the node
        node->data = value;

        // Stores XOR of previous
        // and next pointer
        node->nxp = XOR(NULL, NULL);

        // Update pointer of head node
        *head = node;
    }

    // If the XOR linked list
    // is not empty
    else {

        // Stores the address
        // of current node
        struct Node* curr = *head;

        // Stores the address
        // of previous node
        struct Node* prev = NULL;

        // Initialize a new Node
        struct Node* node
            = (struct Node*)malloc(sizeof(struct Node));

        // Update curr node address
        curr->nxp = XOR(node, XOR(NULL, curr->nxp));

        // Update new node address
        node->nxp = XOR(NULL, curr);

        // Update head
        *head = node;

        // Update data value of
        // current node
        node->data = value;
    }
    return *head;
}

// Function to print elements of
// the XOR Linked List
void printList(struct Node** head)
{

    // Stores XOR pointer
    // in current node
    struct Node* curr = *head;

    // Stores XOR pointer of
    // in previous Node
    struct Node* prev = NULL;

    // Stores XOR pointer of
    // in next node
    struct Node* next;

    // Traverse XOR linked list
    while (curr != NULL) {

        // Print current node
        printf("%d ", curr->data);

        // Forward traversal
        next = XOR(prev, curr->nxp);

        // Update prev
        prev = curr;

        // Update curr
        curr = next;
    }
    printf("\n");
}

// Function to reverse the XOR linked list
struct Node* reverse(struct Node** head)
{

    // Stores XOR pointer
    // in current node
    struct Node* curr = *head;
    if (curr == NULL)
        return NULL;
    else {

        // Stores XOR pointer of
        // in previous Node
        struct Node* prev = NULL;

        // Stores XOR pointer of
        // in next node
        struct Node* next;
        while (XOR(prev, curr->nxp) != NULL) {

            // Forward traversal
            next = XOR(prev, curr->nxp);

            // Update prev
            prev = curr;

            // Update curr
            curr = next;
        }

        // Update the head pointer
        *head = curr;
        return *head;
    }
}

// Driver Code
int main()
{
    /* Create following XOR Linked List
    head-->40<-->30<-->20<-->10 */
    struct Node* head = NULL;
    insert(&head, 10);
    insert(&head, 20);
    insert(&head, 30);
    insert(&head, 40);

    /* Reverse the XOR Linked List to give
    head-->10<-->20<-->30<-->40 */
    printf("XOR linked list: ");
    printList(&head);
    reverse(&head);
    printf("Reversed XOR linked list: ");
    printList(&head);

    return (0);
}

Python3

# Python program for the above approach
import ctypes


# Structure of a node in XOR linked list
class Node:

    def __init__(self, value):
        self.value = value
        self.npx = 0


# create linked list class
class XorLinkedList:

    # constructor
    def __init__(self):
        self.head = None
        self.tail = None
        self.__nodes = []


    # Function to insert a node with given value at given position
    def insert(self, value):

        # Initialize a new Node
        node = Node(value)

        # Check If XOR linked list is empty
        if self.head is None:

            # Update pointer of head node
            self.head = node

            # Update pointer of tail node
            self.tail = node

        else:

            # Update curr node address
            self.head.npx = id(node) ^ self.head.npx

            # Update new node address
            node.npx = id(self.head)

            # Update head
            self.head = node

        # push node
        self.__nodes.append(node)


    # Function to print elements of the XOR Linked List
    def printList(self):


        if self.head != None:
            prev_id = 0
            node = self.head
            next_id = 1
            print(node.value, end=' ')

            # Traverse XOR linked list
            while next_id:

                # Forward traversal
                next_id = prev_id ^ node.npx

                if next_id:

                    # Update prev
                    prev_id = id(node)

                    # Update curr
                    node = self.__type_cast(next_id)

                    # Print current node
                    print(node.value, end=' ')
                else:
                    return


    # method to check if the linked list is empty or not
    def isEmpty(self):
        if self.head is None:
            return True
        return False

    # method to return a new instance of type
    def __type_cast(self, id):
        return ctypes.cast(id, ctypes.py_object).value


    # Function to reverse the XOR linked list
    def reverse(self):

        # Print Values is reverse order.

        if self.head != None:

            # Stores XOR pointer of
            # in previous Node
            prev_id = 0
            node = self.tail

            # Stores XOR pointer of
            # in next node
            next_id = 1

            print(node.value, end=' ')

            while next_id:

                # Forward traversal
                next_id = prev_id ^ node.npx
                if next_id:

                    # Update prev
                    prev_id = id(node)

                    # Update curr
                    node = self.__type_cast(next_id)
                    print(node.value, end=' ')
                else:
                    return


# Create following XOR Linked List
# head-->40<-->30<-->20<-->10
head = XorLinkedList()
head.insert(10)
head.insert(20)
head.insert(30)
head.insert(40)


# Reverse the XOR Linked List to give
# head-->10<-->20<-->30<-->40
print("XOR linked list: ", end = "")
head.printList()

print()
print("Reversed XOR linked list: ", end = "")
head.reverse()

# This code is contributed by Nidhi goel.

Output:

XOR linked list: 40 30 20 10 
Reversed XOR linked list: 10 20 30 40

Time Complexity: O(N)
Auxiliary Space: O(1)