Memory efficient doubly linked list

Last Updated : 29 Aug, 2024

We need to implement a doubly linked list with the use of a single pointer in each node. For that we are given a stream of data of size n for the linked list, your task is to make the function insert() and getList(). The insert() function pushes (or inserts at the beginning) the given data in the linked list and the getList() function returns the linked list as a list.

Note: The List should be printed in both forward and backward direction.

Examples

Input: head= 40<->30<->20<->10
Output: 40 30 20 10
10 20 30 40
Input: head= 5<->4<->3<->2<->1
Output: 5 4 3 2 1
1 2 3 4 5

[Expected Approach] Using Bitwise XOR - O(n) Time and O(1) Space

We know that each node in a doubly-linked list has two pointer fields which contain the addresses of the previous and next node. On the other hand, each node of the XOR linked list requires only a single pointer field, which doesn’t store the actual memory addresses but stores the bitwise XOR of addresses for its previous and next node.

Below is the implementation of the above approach :

C++

// C++ program Implements a doubly linked 
// list using XOR pointers.

#include <bits/stdc++.h>
using namespace std;

class Node {
public:
    int data;
    Node* npx;

    Node(int x) {
        data = x;
        npx = nullptr;
    }
};

// XOR function to get XOR of two pointers
Node* XOR(Node* a, Node* b) {
    return reinterpret_cast<Node*>(reinterpret_cast<uintptr_t>(a) 
           ^ reinterpret_cast<uintptr_t>(b));
}

// Function to insert a node at the front of the list
Node* insert(Node* head, int data) {
  
    // Create a new node with the given data
    Node* new_node = new Node(data);

    // Make the new node's npx point to the head
    new_node->npx = XOR(head, nullptr);

    // Update npx of the head if it's not NULL
    if (head != nullptr) {
        Node* next = XOR(head->npx, nullptr);
        head->npx = XOR(new_node, next);
    }

    // Return the new node as the new head
    return new_node;
}

// Function to retrieve the list as a vector
vector<int> getList(Node* head) {
    vector<int> vec;
    Node* curr = head;
    Node* prev = nullptr;
    Node* next;

    while (curr != nullptr) {
        // Add current node's data to vector
        vec.push_back(curr->data);

        // Calculate the next node using XOR
        next = XOR(prev, curr->npx);

        // Update previous and current nodes
        prev = curr;
        curr = next;
    }

    return vec;
}

int main() {
 
   // Create a hard-coded linked list:
   // 40 <-> 30 <-> 20 <-> 10 (since we insert at the
	// front)
    Node* head = nullptr;

    head = insert(head, 10);
    head = insert(head, 20);
    head = insert(head, 30);
    head = insert(head, 40);

    vector<int> list = getList(head);
  
    for(int i = 0; i < list.size(); ++i) {
        cout<< list[i] <<" ";
    }
    cout << endl;
  
    for(int i = list.size() - 1; i >= 0; --i) {
        cout<< list[i] <<" ";
    }
    cout << endl;

    return 0;
}

// C program Implements a doubly linked 
// list using XOR pointers
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>

struct Node {
    int data;
    struct Node* npx; 
};

struct Node* createNode(int data);

// XOR function to get XOR of two pointers
struct Node* XOR(struct Node* a, struct Node* b) {
    return (struct Node*)((uintptr_t)(a) ^ (uintptr_t)(b));
}


// Function to insert a node at the front of the list
struct Node* insert(struct Node* head, int data) {

    // Create a new node with the given data
    struct Node* new_node = createNode(data);

    // Make the new node's npx point to the head
    new_node->npx = XOR(head, NULL);

    // Update npx of the head if it's not NULL
    if (head != NULL) {
        struct Node* next = XOR(head->npx, NULL);
        head->npx = XOR(new_node, next);
    }

    // Return the new node as the new head
    return new_node;
}

// Function to retrieve the list as an array
void getList(struct Node* head, int* arr, int* len) {
    struct Node* curr = head;
    struct Node* prev = NULL;
    struct Node* next;

    // Initialize array index
    *len = 0;

    while (curr != NULL) {
      
        // Add current node's data to array
        arr[(*len)++] = curr->data;

        // Calculate the next node using XOR
        next = XOR(prev, curr->npx);

        // Update previous and current nodes
        prev = curr;
        curr = next;
    }
}

struct Node* createNode(int data) {
    struct Node* new_node = 
      (struct Node*)malloc(sizeof(struct Node));
    new_node->data = data;
    new_node->npx = NULL;
    return new_node;
}

int main() {
  
  	 // Create a hard-coded linked list:
     // 40 <-> 30 <-> 20 <-> 10 (since we insert at the
     // front)
    struct Node* head = NULL;
    int list[100];
    int len, i;

    head = insert(head, 10);
    head = insert(head, 20);
    head = insert(head, 30);
    head = insert(head, 40);

    getList(head, list, &len);

    for (i = 0; i < len; ++i) {
        printf("%d ", list[i]);
    }
    printf("\n");

    for (i = len - 1; i >= 0; --i) {
        printf("%d ", list[i]);
    }
    printf("\n");

    return 0;
}

Java

// Java program Implements a doubly linked 
// list using XOR pointers
import java.util.ArrayList;

class Node {
    int data;
    Node prev;
    Node next;

    Node(int x) {
        this.data = x;
        this.prev = null;
        this.next = null;
    }
}

public class GfG {

    // Function to insert a node at the front
    static Node insert(Node head, int data) {
      
        // Create a new node with the given data
        Node newNode = new Node(data);

        // Update head's previous pointer if the list is not
        // empty
        if (head != null) {
            head.prev = newNode;
            newNode.next = head;
        }

        // Return the new node as the new head
        return newNode;
    }

    // Function to retrieve the list as an ArrayList
    static ArrayList<Integer> getList(Node head) {
        ArrayList<Integer> list = new ArrayList<>();

        // Start from the head and traverse the list
        Node current = head;
        while (current != null) {
            list.add(current.data);
            current = current.next;
        }

        return list;
    }

    public static void main(String[] args) {
      
        // Create a hard-coded linked list:
        // 40 <-> 30 <-> 20 <-> 10 (since we insert at the
        // front)
        Node head = null;

        head = insert(head, 10);
        head = insert(head, 20);
        head = insert(head, 30);
        head = insert(head, 40);

        ArrayList<Integer> list = getList(head);

        for (int i = 0; i < list.size(); i++) {
            System.out.print(list.get(i) + " ");
        }
        System.out.println();

        for (int i = list.size() - 1; i >= 0; i--) {
            System.out.print(list.get(i) + " ");
        }
        System.out.println();
    }
}

Python

# Python program Implements a doubly linked 
# list using XOR pointers

class Node:
    def __init__(self, data):

        self.data = data
        self.prev = None
        self.next = None

# Function to insert a node at the front
def insert(head, data):
    
    # Create a new node with the given data
    new_node = Node(data)
    
    # Update head's previous pointer if list is not empty
    if head is not None:
        head.prev = new_node
        new_node.next = head
    
    # Return the new node as the new head
    return new_node

# Function to retrieve the list as a list
def getList(head):
    result = []
    
    # Start from the head and traverse the list
    current = head
    while current is not None:
        result.append(current.data)  
        current = current.next 
    
    return result 


if __name__ == "__main__":
  
   # Create a hard-coded linked list:
   # 40 <-> 30 <-> 20 <-> 10 (since we insert at the
   # front)
    head = None

    head = insert(head, 10)
    head = insert(head, 20)
    head = insert(head, 30)
    head = insert(head, 40)

    result_list = getList(head)

    for i in range(len(result_list)):
        print(result_list[i], end=" ")
    print()

    for i in range(len(result_list) - 1, -1, -1):
        print(result_list[i], end=" ")
    print()

// C# program Implements a doubly linked 
// list using XOR pointers

using System;
using System.Collections.Generic;

class Node {
    public int data;
    public Node prev;
    public Node next;
    public Node(int x) {
        data = x; 
        prev = null;
        next = null;
    }
}

class GfG {

    // Function to insert a node at the front
    static Node Insert(Node head, int data) {
        // Create a new node with the given data
        Node newNode = new Node(data);

        // Update head's previous pointer if 
      	// the list is not empty
        if (head != null) {
            head.prev = newNode;
            newNode.next = head;
        }

        // Return the new node as the new head
        return newNode;
    }

    // Function to retrieve the list as a List<int>
    static List<int> GetList(Node head) {
        List<int> result = new List<int>();

        // Start from the head and traverse the list
        Node current = head;
        while (current != null) {
            result.Add(current.data);
            current = current.next;
        }

        return result;
    }

    public static void Main(string[] args) {
      
        // Create a hard-coded linked list:
        // 40 <-> 30 <-> 20 <-> 10 
      	//(since we insert at the front)
        Node head = null;

        head = Insert(head, 10);
        head = Insert(head, 20);
        head = Insert(head, 30);
        head = Insert(head, 40);

        List<int> resultList = GetList(head);
      
        foreach (int num in resultList) {
            Console.Write(num + " ");
        }
        Console.WriteLine();
      
        for (int i = resultList.Count - 1; i >= 0; i--) {
            Console.Write(resultList[i] + " ");
        }
        Console.WriteLine();
    }
}

JavaScript

// Javascript program Implements a doubly linked 
// list using XOR pointers

class Node {
    constructor(data) {

        // Initialize node data and pointers
        this.data = data;
        this.prev = null;
        this.next = null;
    }
}

// Function to insert a node at the front
function insert(head, data) {

    // Create a new node with the given data
    const newNode = new Node(data);

    // Update head's previous pointer if list is not empty
    if (head !== null) {
        head.prev = newNode;
        newNode.next = head;
    }

    // Return the new node as the new head
    return newNode;
}

// Function to retrieve the list as an array
function getList(head) {
    const result = [];

    // Start from the head and traverse the list
    let current = head;
    while (current !== null) {
        result.push(current.data);
        current = current.next;
    }

    return result;
}

// Create a hard-coded linked list:
// 40 <-> 30 <-> 20 <-> 10 (since we insert at the
// front)
let head = null;

head = insert(head, 10);
head = insert(head, 20);
head = insert(head, 30);
head = insert(head, 40);

const resultList = getList(head);

for (let i = 0; i < resultList.length; i++) {
    console.log(resultList[i] + " ");
}

for (let i = resultList.length - 1; i >= 0; i--) {
    console.log(resultList[i] + " ");
}

Output

40 30 20 10 
10 20 30 40

Time Complexity: O(n) for both insertion and retrieval, where n is the number of nodes.
Auxiliary Space: O(1) for insertion, O(n) for storing the list in an array