Reverse a Doubly Linked List GFG problem

 Given a doubly linked list. Your task is to reverse the doubly linked list and return its head.


Examples:


Input: LinkedList: 3 <-> 4 <-> 5

Output: 5 <-> 4 <-> 3


Input: LinkedList: 75 <-> 122 <-> 59 <-> 196

Output: 196 <-> 59 <-> 122 <-> 75


Expected Time Complexity: O(n).

Expected Auxiliary Space: O(1).


//{ Driver Code Starts

import java.io.*;

import java.util.*;


class DLLNode {

    int data;

    DLLNode next;

    DLLNode prev;


    DLLNode(int val) {

        data = val;

        next = null;

        prev = null;

    }

}


public class Main {

    // Function to insert a node at the end of the Doubly Linked List

    public static void push(DLLNode tail, int new_data) {

        DLLNode newNode = new DLLNode(new_data);

        newNode.next = null;

        newNode.prev = tail;


        if (tail != null) {

            tail.next = newNode;

        }

    }


    // Function to print nodes in a given doubly linked list

    public static void printList(DLLNode head) {

        if (head == null) {

            return;

        }


        while (head != null) {

            System.out.print(head.data + " ");

            head = head.next;

        }

        System.out.println();

    }


    // Main driver function

    public static void main(String[] args) throws IOException {

        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

        int t = Integer.parseInt(br.readLine().trim());


        while (t-- > 0) {

            String[] arr = br.readLine().trim().split(" ");


            DLLNode head = new DLLNode(Integer.parseInt(arr[0]));

            DLLNode tail = head;


            for (int i = 1; i < arr.length; i++) {

                push(tail, Integer.parseInt(arr[i]));

                tail = tail.next;

            }


            Solution obj = new Solution();

            head = obj.reverseDLL(head);

            printList(head);

        }

    }


    // Verifying the doubly linked list

    public static boolean verify(DLLNode head) {

        int forwardLength = 0;

        int backwardLength = 0;


        DLLNode temp = head;


        while (temp.next != null) {

            temp = temp.next;

            forwardLength++;

        }


        while (temp.prev != null) {

            temp = temp.prev;

            backwardLength++;

        }


        return forwardLength == backwardLength;

    }

}


// } Driver Code Ends



/*

class DLLNode {

    int data;

    DLLNode next;

    DLLNode prev;


    DLLNode(int val) {

        data = val;

        next = null;

        prev = null;

    }

}

*/

class Solution {

    public DLLNode reverseDLL(DLLNode head) {

        // Your code here

       if (head == null) return null;


        DLLNode current = head;

        DLLNode temp = null;


        // Swap next and prev for each node

        while (current != null) {

            temp = current.prev;

            current.prev = current.next;

            current.next = temp;


            // Move to the next node (which is actually the previous node)

            head = current; // Keep track of last processed node

            current = current.prev;

        }


        return head; // New head of reversed DLL

        

    }

}






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