Given a linked list, reverse alternate nodes and append at the end

Given a linked list, reverse alternate nodes and append them to the end of the list. Extra allowed space is O(1) 

Examples:

Input: 1->2->3->4->5->6
Output: 1->3->5->6->4->2
Explanation: Two lists are 1->3->5 and 2->4->6, 
reverse the 2nd list: 6->4->2. 
Merge the lists 

Input: 12->14->16->18->20
Output: 12->16->20->18->14
Explanation: Two lists are 12->16->20 and 14->18, 
reverse the 2nd list: 18->14. 
Merge the lists 

Approach:

1. The idea is to maintain two linked lists, one list of all odd positioned nodes and other list of all even positioned nodes .
2. Traverse the given linked list which is considered as an odd list or oddly positioned nodes.
3. If the node is even node, remove it from the odd list and add it to the front of even node list. Nodes are added at front to keep the reverse order.
4. Append the even node list at the end of odd node list.

Illustration:

Implementation:

C++

// C++ program to reverse alternate
// nodes of a linked list and append
// at the end
#include <bits/stdc++.h>
using namespace std;

/* A linked list node */
class Node {
public:
    int data;
    Node* next;
};

/* Function to reverse all even positioned
node and append at the end odd is the head
node of given linked list */
void rearrange(Node* odd)
{
    // If linked list has less than 3
    // nodes, no change is required
    if (odd == NULL || odd->next == NULL || odd->next->next == NULL)
        return;

    // even points to the beginning of even list
    Node* even = odd->next;

    // Remove the first even node
    odd->next = odd->next->next;

    // odd points to next node in odd list
    odd = odd->next;

    // Set terminator for even list
    even->next = NULL;

    // Traverse the list
    while (odd->next) {
        // Store the next node in odd list
        Node* temp = odd->next->next;

        // Link the next even node at
        // the beginning of even list
        odd->next->next = even;
        even = odd->next;

        // Remove the even node from middle
        odd->next = temp;

        // Move odd to the next odd node
        if (temp != NULL)
            odd = temp;
    }

    // Append the even list at the end of odd list
    odd->next = even;
}

/* Function to add a node at
the beginning of Linked List */
void push(Node** head_ref, int new_data)
{
    Node* new_node = new Node();
    new_node->data = new_data;
    new_node->next = (*head_ref);
    (*head_ref) = new_node;
}

/* Function to print nodes
in a given linked list */
void printList(Node* node)
{
    while (node != NULL) {
        cout << node->data << " ";
        node = node->next;
    }
}

/* Driver code */
int main()
{
    Node* start = NULL;

    /* The constructed linked list is:
    1->2->3->4->5->6->7 */
    push(&start, 7);
    push(&start, 6);
    push(&start, 5);
    push(&start, 4);
    push(&start, 3);
    push(&start, 2);
    push(&start, 1);

    cout << "Linked list before calling rearrange() ";
    printList(start);

    rearrange(start);

    cout << "\nLinked list after calling rearrange() ";
    printList(start);

    return 0;
}

// This code is contributed by rathbhupendra

C

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

/* A linked list node */
struct Node {
    int data;
    struct Node* next;
};

/* Function to reverse all even positioned
   node and append at the end
   odd is the head node of given linked list */
void rearrange(struct Node* odd)
{
    // If linked list has less than 3 nodes,
    // no change is required
    if (odd == NULL || odd->next == NULL
        || odd->next->next == NULL)
        return;

    // even points to the beginning of even list
    struct Node* even = odd->next;

    // Remove the first even node
    odd->next = odd->next->next;

    // odd points to next node in odd list
    odd = odd->next;

    // Set terminator for even list
    even->next = NULL;

    // Traverse the  list
    while (odd->next) {
        // Store the next node in odd list
        struct Node* temp = odd->next->next;

        // Link the next even node at the
        // beginning of even list
        odd->next->next = even;
        even = odd->next;

        // Remove the even node from middle
        odd->next = temp;

        // Move odd to the next odd node
        if (temp != NULL)
            odd = temp;
    }

    // Append the even list at the end of odd list
    odd->next = even;
}

/* Function to add a node at the
   beginning of Linked List */
void push(struct Node** head_ref,
          int new_data)
{
    struct Node* new_node
        = (struct Node*)malloc(
            sizeof(struct Node));
    new_node->data = new_data;
    new_node->next = (*head_ref);
    (*head_ref) = new_node;
}

/* Function to print nodes in a
   given linked list */
void printList(struct Node* node)
{
    while (node != NULL) {
        printf("%d ", node->data);
        node = node->next;
    }
}

/* Driver program to test above function */
int main()
{
    struct Node* start = NULL;

    /* The constructed linked list is:
     1->2->3->4->5->6->7 */
    push(&start, 7);
    push(&start, 6);
    push(&start, 5);
    push(&start, 4);
    push(&start, 3);
    push(&start, 2);
    push(&start, 1);

    printf("\n Linked list before calling  rearrange() ");
    printList(start);

    rearrange(start);

    printf("\n Linked list after calling  rearrange() ");
    printList(start);

    return 0;
}

Java

// Java program to reverse alternate
// nodes of a linked list and append
// at the end

class LinkedList {

    static Node head;

    static class Node {

        int data;
        Node next;

        Node(int item)
        {
            data = item;
            next = null;
        }
    }

    /* Function to reverse all even
       positioned node and append at the end
       odd is the head node of given linked list */
    void rearrange(Node odd)
    {

        // If linked list has less than 3 nodes,
        // no change is required
        if (odd == null || odd.next == null
            || odd.next.next == null) {
            return;
        }

        // even points to the beginning
        // of even list
        Node even = odd.next;

        // Remove the first even node
        odd.next = odd.next.next;

        // odd points to next node in odd list
        odd = odd.next;

        // Set terminator for even list
        even.next = null;

        // Traverse the  list
        while (odd.next != null) {

            // Store the next node in odd list
            Node temp = odd.next.next;

            // Link the next even node at the
            // beginning of even list
            odd.next.next = even;
            even = odd.next;

            // Remove the even node from middle
            odd.next = temp;

            // Move odd to the next odd node
            if (temp != null) {
                odd = temp;
            }
        }

        // Append the even list at the end of odd list
        odd.next = even;
    }

    /* Function to print nodes in a given linked list */
    void printList(Node node)
    {
        while (node != null) {
            System.out.print(node.data + " ");
            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.head.next.next.next.next.next = new Node(6);
        list.head.next.next.next.next.next.next = new Node(7);

        System.out.println("Linked list before calling rearrange : ");
        list.printList(head);

        System.out.println("");
        list.rearrange(head);

        System.out.println("Linked list after calling rearrange : ");
        list.printList(head);
    }
}

Python3

# Python program to reverse alternate nodes and append
# at end
# Extra space allowed - O(1)

# Node Class
class Node:

    # Constructor to initialize the node object
    def  __init__(self, data):
        self.data = data
        self.next = None

# Linked list class contains node object
class LinkedList:

    # Constructor to initialize head
    def __init__(self):
        self.head = None

    # Function to insert a new node at the beginning
    def push(self, new_data):
        new_node = Node(new_data)
        new_node.next = self.head
        self.head = new_node


    def printList(self):
        temp = self.head
        while(temp):
            print (temp.data,end=" ")
            temp = temp.next

    def rearrange(self):

        # If linked list has less than 3 nodes, no change
        # is required
        odd = self.head
        if (odd is None or odd.next is None or
            odd.next.next is None):
            return

        # Even points to the beginning of even list
        even = odd.next

        # Remove the first even node
        odd.next = odd.next.next

        # Odd points to next node in odd list
        odd = odd.next

        # Set terminator for even list
        even.next = None

        # Traverse the list
        while (odd.next):
            # Store the next node in odd list
            temp = odd.next.next

            # Link the next even node at the beginning
            # of even list
            odd.next.next = even
            even = odd.next

            # Remove the even node from middle
            odd.next = temp

            # Move odd to the next odd node
            if temp is not None:
                odd = temp

        # Append the even list at the end of odd list
        odd.next = even

# Code execution starts here
if __name__ == '__main__':
    start = LinkedList()

    # The constructed linked list is ;
    # 1->2->3->4->5->6->7
    start.push(7)
    start.push(6)
    start.push(5)
    start.push(4)
    start.push(3)
    start.push(2)
    start.push(1)

    print ("Linked list before calling  rearrange() ")
    start.printList()

    start.rearrange()

    print ("\nLinked list after calling  rearrange()")
    start.printList()

# This code is contributed by NIkhil Kumar Singh(nickzuck_007)

C#

// C# program to reverse alternate
// nodes of a linked list
// and append at the end
using System;

public class LinkedList {

    Node head;

    public class Node {

        public int data;
        public Node next;

        public Node(int item)
        {
            data = item;
            next = null;
        }
    }

    /* Function to reverse all even
    positioned node and append at the end
    odd is the head node of given linked list */
    void rearrange(Node odd)
    {

        // If linked list has less than 3
        // nodes, no change is required
        if (odd == null || odd.next == null || odd.next.next == null) {
            return;
        }

        // even points to the beginning of even list
        Node even = odd.next;

        // Remove the first even node
        odd.next = odd.next.next;

        // odd points to next node in odd list
        odd = odd.next;

        // Set terminator for even list
        even.next = null;

        // Traverse the list
        while (odd.next != null) {

            // Store the next node in odd list
            Node temp = odd.next.next;

            // Link the next even node at
            // the beginning of even list
            odd.next.next = even;
            even = odd.next;

            // Remove the even node from middle
            odd.next = temp;

            // Move odd to the next odd node
            if (temp != null) {
                odd = temp;
            }
        }

        // Append the even list at the end of odd list
        odd.next = even;
    }

    /* Function to print nodes in a given linked list */
    void printList(Node node)
    {
        while (node != null) {
            Console.Write(node.data + " ");
            node = node.next;
        }
    }

    // Driver code
    public static void Main()
    {
        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.head.next.next.next.next.next = new Node(6);
        list.head.next.next.next.next.next.next = new Node(7);

        Console.WriteLine("Linked list before calling rearrange : ");
        list.printList(list.head);

        Console.WriteLine("");
        list.rearrange(list.head);

        Console.WriteLine("Linked list after calling rearrange : ");
        list.printList(list.head);
    }
}

/* This code contributed by PrinciRaj1992 */

Javascript

<script>

// Javascript program to reverse alternate
// nodes of a linked list and append
// at the end
class Node
{
    constructor(item)
    {
        this.data = item;
        this.next = null;
    }
}

let head;

// Function to reverse all even positioned
// node and append at the end odd is the
// head node of given linked list
function rearrange(odd)
{

    // If linked list has less than 3 nodes,
    // no change is required
    if (odd == null || odd.next == null ||
        odd.next.next == null)
    {
        return;
    }

    // Even points to the beginning
    // of even list
    let even = odd.next;

    // Remove the first even node
    odd.next = odd.next.next;

    // Odd points to next node in odd list
    odd = odd.next;

    // Set terminator for even list
    even.next = null;

    // Traverse the  list
    while (odd.next != null)
    {

        // Store the next node in odd list
        let temp = odd.next.next;

        // Link the next even node at the
        // beginning of even list
        odd.next.next = even;
        even = odd.next;

        // Remove the even node from middle
        odd.next = temp;

        // Move odd to the next odd node
        if (temp != null)
        {
            odd = temp;
        }
    }

    // Append the even list at the
    // end of odd list
    odd.next = even;
}

// Function to print nodes in a
// given linked list
function printList(node)
{
    while (node != null)
    {
        document.write(node.data + " ");
        node = node.next;
    }
}

// Driver code
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);
head.next.next.next.next.next = new Node(6);
head.next.next.next.next.next.next = new Node(7);

document.write("Linked list before " +
               "calling rearrange : <br>");
printList(head);

document.write("<br>");
rearrange(head);

document.write("Linked list after " +
               "calling rearrange : <br>");
printList(head);

// This code is contributed by avanitrachhadiya2155

</script>

Output

Linked list before calling rearrange() 1 2 3 4 5 6 7 
Linked list after calling rearrange() 1 3 5 7 6 4 2 

Complexity Analysis:

• Time Complexity: O(n). 
  The above code simply traverses the given linked list. So time complexity is O(n)
• Auxiliary Space: O(1). 
  No extra space is required.