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Rearrange a given linked list in-place.
  • Difficulty Level : Medium
  • Last Updated : 03 Jun, 2021

Given a singly linked list L0 -> L1 -> … -> Ln-1 -> Ln. Rearrange the nodes in the list so that the new formed list is : L0 -> Ln -> L1 -> Ln-1 -> L2 -> Ln-2
You are required to do this in place without altering the nodes’ values. 

Examples: 

Input:  1 -> 2 -> 3 -> 4
Output: 1 -> 4 -> 2 -> 3

Input:  1 -> 2 -> 3 -> 4 -> 5
Output: 1 -> 5 -> 2 -> 4 -> 3

Simple Solution

1) Initialize current node as head.
2) While next of current node is not null, do following
    a) Find the last node, remove it from the end and insert it as next
       of the current node.
    b) Move current to next to next of current

The time complexity of the above simple solution is O(n2) where n is the number of nodes in the linked list.

Better Solution 
1) Copy contents of the given linked list to a vector. 
2) Rearrange the given vector by swapping nodes from both ends. 
3) Copy the modified vector back to the linked list. 
Implementation of this approach: https://ide.geeksforgeeks.org/1eGSEy 
Thanks to Arushi Dhamija for suggesting this approach.



Efficient Solution:

1) Find the middle point using tortoise and hare method.
2) Split the linked list into two halves using found middle point in step 1.
3) Reverse the second half.
4) Do alternate merge of first and second halves.

The Time Complexity of this solution is O(n). 

Below is the implementation of this method.

C++




// C++ program to rearrange a linked list in-place
#include <bits/stdc++.h>
using namespace std;
 
// Linkedlist Node structure
struct Node {
    int data;
    struct Node* next;
};
 
// Function to create newNode in a linkedlist
Node* newNode(int key)
{
    Node* temp = new Node;
    temp->data = key;
    temp->next = NULL;
    return temp;
}
 
// Function to reverse the linked list
void reverselist(Node** head)
{
    // Initialize prev and current pointers
    Node *prev = NULL, *curr = *head, *next;
 
    while (curr) {
        next = curr->next;
        curr->next = prev;
        prev = curr;
        curr = next;
    }
 
    *head = prev;
}
 
// Function to print the linked list
void printlist(Node* head)
{
    while (head != NULL) {
        cout << head->data << " ";
        if (head->next)
            cout << "-> ";
        head = head->next;
    }
    cout << endl;
}
 
// Function to rearrange a linked list
void rearrange(Node** head)
{
    // 1) Find the middle point using tortoise and hare
    // method
    Node *slow = *head, *fast = slow->next;
    while (fast && fast->next) {
        slow = slow->next;
        fast = fast->next->next;
    }
 
    // 2) Split the linked list in two halves
    // head1, head of first half    1 -> 2
    // head2, head of second half   3 -> 4
    Node* head1 = *head;
    Node* head2 = slow->next;
    slow->next = NULL;
 
    // 3) Reverse the second half, i.e.,  4 -> 3
    reverselist(&head2);
 
    // 4) Merge alternate nodes
    *head = newNode(0); // Assign dummy Node
 
    // curr is the pointer to this dummy Node, which will
    // be used to form the new list
    Node* curr = *head;
    while (head1 || head2) {
        // First add the element from list
        if (head1) {
            curr->next = head1;
            curr = curr->next;
            head1 = head1->next;
        }
 
        // Then add the element from the second list
        if (head2) {
            curr->next = head2;
            curr = curr->next;
            head2 = head2->next;
        }
    }
 
    // Assign the head of the new list to head pointer
    *head = (*head)->next;
}
 
// Driver program
int main()
{
    Node* head = newNode(1);
    head->next = newNode(2);
    head->next->next = newNode(3);
    head->next->next->next = newNode(4);
    head->next->next->next->next = newNode(5);
 
    printlist(head); // Print original list
    rearrange(&head); // Modify the list
    printlist(head); // Print modified list
    return 0;
}

Java




// Java program to rearrange link list in place
 
// Linked List Class
class LinkedList {
 
    static Node head; // head of the list
 
    /* Node Class */
    static class Node {
 
        int data;
        Node next;
 
        // Constructor to create a new node
        Node(int d)
        {
            data = d;
            next = null;
        }
    }
 
    void printlist(Node node)
    {
        if (node == null) {
            return;
        }
        while (node != null) {
            System.out.print(node.data + " -> ");
            node = node.next;
        }
    }
 
    Node reverselist(Node node)
    {
        Node prev = null, curr = node, next;
        while (curr != null) {
            next = curr.next;
            curr.next = prev;
            prev = curr;
            curr = next;
        }
        node = prev;
        return node;
    }
 
    void rearrange(Node node)
    {
 
        // 1) Find the middle point using tortoise and hare
        // method
        Node slow = node, fast = slow.next;
        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
 
        // 2) Split the linked list in two halves
        // node1, head of first half    1 -> 2 -> 3
        // node2, head of second half   4 -> 5
        Node node1 = node;
        Node node2 = slow.next;
        slow.next = null;
 
        // 3) Reverse the second half, i.e., 5 -> 4
        node2 = reverselist(node2);
 
        // 4) Merge alternate nodes
        node = new Node(0); // Assign dummy Node
 
        // curr is the pointer to this dummy Node, which
        // will be used to form the new list
        Node curr = node;
        while (node1 != null || node2 != null) {
 
            // First add the element from first list
            if (node1 != null) {
                curr.next = node1;
                curr = curr.next;
                node1 = node1.next;
            }
 
            // Then add the element from second list
            if (node2 != null) {
                curr.next = node2;
                curr = curr.next;
                node2 = node2.next;
            }
        }
 
        // Assign the head of the new list to head pointer
        node = node.next;
    }
 
    public static void main(String[] args)
    {
 
        LinkedList list = new LinkedList();
        list.head = new Node(1);
        list.head.next = new Node(2);
        list.head.next.next = new Node(3);
        list.head.next.next.next = new Node(4);
        list.head.next.next.next.next = new Node(5);
 
        list.printlist(head); // print original list
        list.rearrange(head); // rearrange list as per ques
        System.out.println("");
        list.printlist(head); // print modified list
    }
}
 
// This code has been contributed by Mayank Jaiswal

C#




// C# program to rearrange link list in place
using System;
 
// Linked List Class
public class LinkedList {
 
    Node head; // head of the list
 
    /* Node Class */
    class Node {
 
        public int data;
        public Node next;
 
        // Constructor to create a new node
        public Node(int d)
        {
            data = d;
            next = null;
        }
    }
 
    void printlist(Node node)
    {
        if (node == null) {
            return;
        }
        while (node != null) {
            Console.Write(node.data + " -> ");
            node = node.next;
        }
    }
 
    Node reverselist(Node node)
    {
        Node prev = null, curr = node, next;
        while (curr != null) {
            next = curr.next;
            curr.next = prev;
            prev = curr;
            curr = next;
        }
        node = prev;
        return node;
    }
 
    void rearrange(Node node)
    {
 
        // 1) Find the middle point using
        // tortoise and hare method
        Node slow = node, fast = slow.next;
        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
 
        // 2) Split the linked list in two halves
        // node1, head of first half 1 -> 2 -> 3
        // node2, head of second half 4 -> 5
        Node node1 = node;
        Node node2 = slow.next;
        slow.next = null;
 
        // 3) Reverse the second half, i.e., 5 -> 4
        node2 = reverselist(node2);
 
        // 4) Merge alternate nodes
        node = new Node(0); // Assign dummy Node
 
        // curr is the pointer to this dummy Node, which
        // will be used to form the new list
        Node curr = node;
        while (node1 != null || node2 != null) {
 
            // First add the element from first list
            if (node1 != null) {
                curr.next = node1;
                curr = curr.next;
                node1 = node1.next;
            }
 
            // Then add the element from second list
            if (node2 != null) {
                curr.next = node2;
                curr = curr.next;
                node2 = node2.next;
            }
        }
 
        // Assign the head of the new list to head pointer
        node = node.next;
    }
 
    // Driver code
    public static void main(String[] args)
    {
 
        LinkedList list = new LinkedList();
        list.head = new Node(1);
        list.head.next = new Node(2);
        list.head.next.next = new Node(3);
        list.head.next.next.next = new Node(4);
        list.head.next.next.next.next = new Node(5);
 
        list.printlist(list.head); // print original list
        list.rearrange(
            list.head); // rearrange list as per ques
        Console.WriteLine("");
        list.printlist(list.head); // print modified list
    }
}
 
/* This code is contributed PrinciRaj1992 */

Javascript




<script>
 
// Javascript program to rearrange link list in place
 
// Linked List Class
var head; // head of the list
 
    /* Node Class */
     class Node {
 
// Constructor to create a new node
constructor(d) {
    this.data = d;
    this.next = null;
}
 
    }
 
    function printlist(node) {
        if (node == null) {
            return;
        }
        while (node != null) {
            document.write(node.data + " -> ");
            node = node.next;
        }
    }
 
    function reverselist(node) {
        var prev = null, curr = node, next;
        while (curr != null) {
            next = curr.next;
            curr.next = prev;
            prev = curr;
            curr = next;
        }
        node = prev;
        return node;
    }
 
    function rearrange(node) {
 
        // 1) Find the middle point using tortoise and hare
        // method
        var slow = node, fast = slow.next;
        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
 
        // 2) Split the linked list in two halves
        // node1, head of first half 1 -> 2 -> 3
        // node2, head of second half 4 -> 5
        var node1 = node;
        var node2 = slow.next;
        slow.next = null;
 
        // 3) Reverse the second half, i.e., 5 -> 4
        node2 = reverselist(node2);
 
        // 4) Merge alternate nodes
        node = new Node(0); // Assign dummy Node
 
        // curr is the pointer to this dummy Node, which
        // will be used to form the new list
        var curr = node;
        while (node1 != null || node2 != null) {
 
            // First add the element from first list
            if (node1 != null) {
                curr.next = node1;
                curr = curr.next;
                node1 = node1.next;
            }
 
            // Then add the element from second list
            if (node2 != null) {
                curr.next = node2;
                curr = curr.next;
                node2 = node2.next;
            }
        }
 
        // Assign the head of the new list to head pointer
        node = node.next;
    }
 
     
        head = new Node(1);
        head.next = new Node(2);
        head.next.next = new Node(3);
        head.next.next.next = new Node(4);
        head.next.next.next.next = new Node(5);
 
        printlist(head); // print original list
        rearrange(head); // rearrange list as per ques
        document.write("<br/>");
        printlist(head); // print modified list
 
// This code contributed by gauravrajput1
</script>
Output: 
1 -> 2 -> 3 -> 4 -> 5 
1 -> 5 -> 2 -> 4 -> 3

 

Time Complexity: O(n) 
Auxiliary Space: O(1)
Thanks to Gaurav Ahirwar for suggesting the above approach.

Another approach: 
1. Take two pointers prev and curr, which hold the addresses of head and head-> next. 
2. Compare their data and swap. 
After that, a new linked list is formed. 

Below is the implementation: 

C++




// C++ code to rearrange linked list in place
#include <bits/stdc++.h>
 
using namespace std;
 
struct node {
    int data;
    struct node* next;
};
typedef struct node Node;
 
// function for rearranging a linked list with high and low
// value.
void rearrange(Node* head)
{
    if (head == NULL) // Base case.
        return;
 
    // two pointer variable.
    Node *prev = head, *curr = head->next;
 
    while (curr) {
        // swap function for swapping data.
        if (prev->data > curr->data)
            swap(prev->data, curr->data);
 
        // swap function for swapping data.
        if (curr->next && curr->next->data > curr->data)
            swap(curr->next->data, curr->data);
 
        prev = curr->next;
 
        if (!curr->next)
            break;
        curr = curr->next->next;
    }
}
 
// function to insert a node in the linked list at the
// beginning.
void push(Node** head, int k)
{
    Node* tem = (Node*)malloc(sizeof(Node));
    tem->data = k;
    tem->next = *head;
    *head = tem;
}
 
// function to display node of linked list.
void display(Node* head)
{
    Node* curr = head;
    while (curr != NULL) {
        printf("%d ", curr->data);
        curr = curr->next;
    }
}
 
// driver code
int main()
{
 
    Node* head = NULL;
 
    // let create a linked list.
    // 9 -> 6 -> 8 -> 3 -> 7
    push(&head, 7);
    push(&head, 3);
    push(&head, 8);
    push(&head, 6);
    push(&head, 9);
 
    rearrange(head);
 
    display(head);
 
    return 0;
}

Java




// Java code to rearrange linked list in place
class Geeks {
 
    static class Node {
        int data;
        Node next;
    }
 
    // function for rearranging a linked list
    // with high and low value.
    static Node rearrange(Node head)
    {
        if (head == null) // Base case.
            return null;
 
        // two pointer variable.
        Node prev = head, curr = head.next;
 
        while (curr != null) {
            // swap function for swapping data.
            if (prev.data > curr.data) {
                int t = prev.data;
                prev.data = curr.data;
                curr.data = t;
            }
 
            // swap function for swapping data.
            if (curr.next != null
                && curr.next.data > curr.data) {
                int t = curr.next.data;
                curr.next.data = curr.data;
                curr.data = t;
            }
 
            prev = curr.next;
 
            if (curr.next == null)
                break;
            curr = curr.next.next;
        }
        return head;
    }
 
    // function to insert a Node in
    // the linked list at the beginning.
    static Node push(Node head, int k)
    {
        Node tem = new Node();
        tem.data = k;
        tem.next = head;
        head = tem;
        return head;
    }
 
    // function to display Node of linked list.
    static void display(Node head)
    {
        Node curr = head;
        while (curr != null) {
            System.out.printf("%d ", curr.data);
            curr = curr.next;
        }
    }
 
    // Driver code
    public static void main(String args[])
    {
 
        Node head = null;
 
        // let create a linked list.
        // 9 . 6 . 8 . 3 . 7
        head = push(head, 7);
        head = push(head, 3);
        head = push(head, 8);
        head = push(head, 6);
        head = push(head, 9);
 
        head = rearrange(head);
 
        display(head);
    }
}
 
// This code is contributed by Arnab Kundu

Python3




# Python3 code to rearrange linked list in place
class Node:
 
    def __init__(self, x):
 
        self.data = x
        self.next = None
 
# Function for rearranging a linked
# list with high and low value
 
 
def rearrange(head):
 
    # Base case
    if (head == None):
        return head
 
    # Two pointer variable
    prev, curr = head, head.next
 
    while (curr):
 
        # Swap function for swapping data
        if (prev.data > curr.data):
            prev.data, curr.data = curr.data, prev.data
 
        # Swap function for swapping data
        if (curr.next and curr.next.data > curr.data):
            curr.next.data, curr.data = curr.data, curr.next.data
 
        prev = curr.next
 
        if (not curr.next):
            break
 
        curr = curr.next.next
 
    return head
 
# Function to insert a node in the
# linked list at the beginning
 
 
def push(head, k):
 
    tem = Node(k)
    tem.data = k
    tem.next = head
    head = tem
    return head
 
# Function to display node of linked list
 
 
def display(head):
 
    curr = head
 
    while (curr != None):
        print(curr.data, end=" ")
        curr = curr.next
 
 
# Driver code
if __name__ == '__main__':
 
    head = None
 
    # Let create a linked list
    # 9 . 6 . 8 . 3 . 7
    head = push(head, 7)
    head = push(head, 3)
    head = push(head, 8)
    head = push(head, 6)
    head = push(head, 9)
 
    head = rearrange(head)
 
    display(head)
 
# This code is contributed by mohit kumar 29

C#




// C# code to rearrange linked list in place
using System;
 
class GFG {
 
    class Node {
        public int data;
        public Node next;
    }
 
    // Function for rearranging a linked list
    // with high and low value.
    static Node rearrange(Node head)
    {
 
        // Base case
        if (head == null)
            return null;
 
        // Two pointer variable.
        Node prev = head, curr = head.next;
 
        while (curr != null) {
 
            // Swap function for swapping data.
            if (prev.data > curr.data) {
                int t = prev.data;
                prev.data = curr.data;
                curr.data = t;
            }
 
            // Swap function for swapping data.
            if (curr.next != null
                && curr.next.data > curr.data) {
                int t = curr.next.data;
                curr.next.data = curr.data;
                curr.data = t;
            }
 
            prev = curr.next;
 
            if (curr.next == null)
                break;
 
            curr = curr.next.next;
        }
        return head;
    }
 
    // Function to insert a Node in
    // the linked list at the beginning.
    static Node push(Node head, int k)
    {
        Node tem = new Node();
        tem.data = k;
        tem.next = head;
        head = tem;
        return head;
    }
 
    // Function to display Node of linked list.
    static void display(Node head)
    {
        Node curr = head;
 
        while (curr != null) {
            Console.Write(curr.data + " ");
            curr = curr.next;
        }
    }
 
    // Driver code
    public static void Main(string[] args)
    {
        Node head = null;
 
        // Let create a linked list.
        // 9 . 6 . 8 . 3 . 7
        head = push(head, 7);
        head = push(head, 3);
        head = push(head, 8);
        head = push(head, 6);
        head = push(head, 9);
 
        head = rearrange(head);
 
        display(head);
    }
}
 
// This code is contributed by rutvik_56

Javascript




<script>
// Javascript code to rearrange linked list in place
     
    class Node
    {
        constructor()
        {
            this.data;
            this.next=null;
        }
    }
     
    // function for rearranging a linked list
    // with high and low value.
    function rearrange(head)
    {
        if (head == null) // Base case.
            return null;
  
        // two pointer variable.
        let prev = head, curr = head.next;
  
        while (curr != null)
        {
         
            // swap function for swapping data.
            if (prev.data > curr.data) {
                let t = prev.data;
                prev.data = curr.data;
                curr.data = t;
            }
  
            // swap function for swapping data.
            if (curr.next != null
                && curr.next.data > curr.data) {
                let t = curr.next.data;
                curr.next.data = curr.data;
                curr.data = t;
            }
  
            prev = curr.next;
  
            if (curr.next == null)
                break;
            curr = curr.next.next;
        }
        return head;
    }
     
    // function to display Node of linked list.
    function display(head)
    {
        let curr = head;
        while (curr != null) {
            document.write(curr.data+" ");
            curr = curr.next;
        }
    }
     
    // function to insert a Node in
    // the linked list at the beginning.
    function push(head,k)
    {
        let tem = new Node();
        tem.data = k;
        tem.next = head;
        head = tem;
        return head;
    }
     
     
    // Driver code
     
    let head = null;
    head = push(head, 7);
    head = push(head, 3);
    head = push(head, 8);
    head = push(head, 6);
    head = push(head, 9);
    head = rearrange(head);
    display(head);
     
    // This code is contributed by unknown2108
</script>

 
 



Output: 
6 9 3 8 7

 

 

Time Complexity : O(n) 
Auxiliary Space : O(1) 
Thanks to Aditya for suggesting this approach.

 

Another Approach: (Using recursion) 

 

  1. Hold a pointer to the head node and go till the last node using recursion
  2. Once the last node is reached, start swapping the last node to the next of head node
  3. Move the head pointer to the next node
  4. Repeat this until the head and the last node meet or come adjacent to each other
  5. Once the Stop condition met, we need to discard the left nodes to fix the loop created in the list while swapping nodes.

 

C




// C/C++ implementation
#include <stdio.h>
#include <stdlib.h>
 
// Creating the structure for node
struct Node {
    int data;
    struct Node* next;
};
 
// Function to create newNode in a linkedlist
struct Node* newNode(int key)
{
    struct Node* temp = malloc(sizeof(struct Node));
    temp->data = key;
    temp->next = NULL;
    return temp;
}
 
// Function to print the list
void printlist(struct Node* head)
{
    while (head) {
        printf("%d ", head->data);
        if (head->next)
            printf("->");
        head = head->next;
    }
    printf("\n");
}
 
// Function to rearrange
void rearrange(struct Node** head, struct Node* last)
{
 
    if (!last)
        return;
 
    // Recursive call
    rearrange(head, last->next);
 
    // (*head)->next will be set to NULL
    // after rearrangement.
    // Need not do any operation further
    // Just return here to come out of recursion
    if (!(*head)->next)
        return;
 
    // Rearrange the list until both head
    // and last meet or next to each other.
    if ((*head) != last && (*head)->next != last) {
        struct Node* tmp = (*head)->next;
        (*head)->next = last;
        last->next = tmp;
        *head = tmp;
    }
    else {
        if ((*head) != last)
            *head = (*head)->next;
        (*head)->next = NULL;
    }
}
 
// Drivers Code
int main()
{
    struct Node* head = newNode(1);
    head->next = newNode(2);
    head->next->next = newNode(3);
    head->next->next->next = newNode(4);
    head->next->next->next->next = newNode(5);
 
    // Print original list
    printlist(head);
 
    struct Node* tmp = head;
 
    // Modify the list
    rearrange(&tmp, head);
 
    // Print modified list
    printlist(head);
    return 0;
}

Java




// Java implementation
import java.io.*;
 
// Creating the structure for node
class Node {
    int data;
    Node next;
 
    // Function to create newNode in a linkedlist
    Node(int key)
    {
        data = key;
        next = null;
    }
}
class GFG {
 
    Node left = null;
 
    // Function to print the list
    void printlist(Node head)
    {
        while (head != null) {
            System.out.print(head.data + " ");
            if (head.next != null) {
                System.out.print("->");
            }
            head = head.next;
        }
        System.out.println();
    }
 
    // Function to rearrange
    void rearrange(Node head)
    {
 
        if (head != null) {
            left = head;
            reorderListUtil(left);
        }
    }
 
    void reorderListUtil(Node right)
    {
 
        if (right == null) {
            return;
        }
 
        reorderListUtil(right.next);
 
        // we set left = null, when we reach stop condition,
        // so no processing required after that
        if (left == null) {
            return;
        }
 
        // Stop condition: odd case : left = right, even
        // case : left.next = right
        if (left != right && left.next != right) {
            Node temp = left.next;
            left.next = right;
            right.next = temp;
            left = temp;
        }
        else { // stop condition , set null to left nodes
            if (left.next == right) {
                left.next.next = null; // even case
                left = null;
            }
            else {
                left.next = null; // odd case
                left = null;
            }
        }
    }
 
    // Drivers Code
    public static void main(String[] args)
    {
 
        Node head = new Node(1);
        head.next = new Node(2);
        head.next.next = new Node(3);
        head.next.next.next = new Node(4);
        head.next.next.next.next = new Node(5);
       
        GFG gfg = new GFG();
 
        // Print original list
        gfg.printlist(head);       
 
        // Modify the list
        gfg.rearrange(head);
 
        // Print modified list
        gfg.printlist(head);
    }
}
 
// This code is contributed by Vishal Singh

C#




// C# implementation
using System;
 
// Creating the structure for node
public class Node
{
    public int data;
    public Node next;
     
    // Function to create newNode
    // in a linkedlist
    public Node(int key)
    {
        data = key;
        next = null;
    }
}
 
class GFG{
     
Node left = null;
 
// Function to print the list
void printlist(Node head)
{
    while (head != null)
    {
        Console.Write(head.data + " ");
        if (head.next != null)
        {
            Console.Write("->");
        }
        head = head.next;
    }
    Console.WriteLine();
}
 
// Function to rearrange
void rearrange(Node head)
{
 
    if (head != null)
    {
        left = head;
        reorderListUtil(left);
    }
}
 
void reorderListUtil(Node right)
{
 
    if (right == null)
    {
        return;
    }
 
    reorderListUtil(right.next);
 
    // We set left = null, when we reach stop
    // condition, so no processing required
    // after that
    if (left == null)
    {
        return;
    }
 
    // Stop condition: odd case : left = right, even
    // case : left.next = right
    if (left != right && left.next != right)
    {
        Node temp = left.next;
        left.next = right;
        right.next = temp;
        left = temp;
    }
    else
    {
         
        // Stop condition , set null to left nodes
        if (left.next == right)
        {
             
            // Even case
            left.next.next = null;
            left = null;
        }
        else
        {
             
            // Odd case
            left.next = null;
            left = null;
        }
    }
}
 
// Driver Code
static public void Main()
{
    Node head = new Node(1);
    head.next = new Node(2);
    head.next.next = new Node(3);
    head.next.next.next = new Node(4);
    head.next.next.next.next = new Node(5);
    
    GFG gfg = new GFG();
 
    // Print original list
    gfg.printlist(head);       
 
    // Modify the list
    gfg.rearrange(head);
 
    // Print modified list
    gfg.printlist(head);
}
}
 
// This code is contributed by rag2127

Javascript




<script>
// javascript implementation// Creating the structure for node
class Node {
 
    // Function to create newNode in a linkedlist
    constructor(val) {
        this.data = val;
        this.next = null;
    }
}
    var left = null;
 
    // Function to print the list
    function printlist(head) {
        while (head != null) {
            document.write(head.data + " ");
            if (head.next != null) {
                document.write("->");
            }
            head = head.next;
        }
        document.write("<br/>");
    }
 
    // Function to rearrange
    function rearrange(head) {
 
        if (head != null) {
            left = head;
            reorderListUtil(left);
        }
    }
 
    function reorderListUtil(right) {
 
        if (right == null) {
            return;
        }
 
        reorderListUtil(right.next);
 
        // we set left = null, when we reach stop condition,
        // so no processing required after that
        if (left == null) {
            return;
        }
 
        // Stop condition: odd case : left = right, even
        // case : left.next = right
        if (left != right && left.next != right) {
    var temp = left.next;
            left.next = right;
            right.next = temp;
            left = temp;
        } else { // stop condition , set null to left nodes
            if (left.next == right) {
                left.next.next = null; // even case
                left = null;
            } else {
                left.next = null; // odd case
                left = null;
            }
        }
    }
 
    // Drivers Code
     
 
var head = new Node(1);
        head.next = new Node(2);
        head.next.next = new Node(3);
        head.next.next.next = new Node(4);
        head.next.next.next.next = new Node(5);
 
 
        // Print original list
        printlist(head);
 
        // Modify the list
        rearrange(head);
 
        // Print modified list
        printlist(head);
 
// This code contributed by aashish1995
</script>
Output: 
1 ->2 ->3 ->4 ->5 
1 ->5 ->2 ->4 ->3

 

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