Merge first half and reversed second half of the linked list alternatively

Given a linked list, the task is to rearrange the linked list in the following manner: 

1. Reverse the second half of given linked list.
2. • First element of the linked list is the first element of first half.
   • Second element of the linked list is the first element of second half.

Examples:

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

Input: 1->2->3->4->5->6
Output: 1->6->2->5->3->4

Approach: 

Initially find the mid node of the linked list. The approach has been discussed here. Reverse the linked list from mid to end. Once the linked list is reversed, traverse from the start and insert a node from the first half of the list and another node from the back half of the linked list simultaneously. Continue this process until the middle node is reached.

Once the middle node is reached, point the node just before the middle node to NULL.

Below is the implementation of the above approach:  

C++

// C++ program to sandwich the last part of
// linked list in between the first part of
// the linked list
#include<bits/stdc++.h>
#include <stdio.h>
using namespace std;

struct node {
    int data;
    struct node* next;
};

// Function to reverse Linked List
struct node* reverseLL(struct node* root)
{
    struct node *prev = NULL, *current = root, *next;
    while (current) {
        next = current->next;
        current->next = prev;
        prev = current;
        current = next;
    }

    // root needs to be updated after reversing
    root = prev;
    return root;
}

// Function to modify Linked List
void modifyLL(struct node* root)
{
    // Find the mid node
    struct node *slow_ptr = root, *fast_ptr = root;
    while (fast_ptr && fast_ptr->next) {
        fast_ptr = fast_ptr->next->next;
        slow_ptr = slow_ptr->next;
    }

    // Reverse the second half of the list
    struct node* root2 = reverseLL(slow_ptr->next);

    // partition the list
    slow_ptr->next = NULL;

    struct node *current1 = root, *current2 = root2;

    // insert the elements in between
    while (current1 && current2) {

        // next node to be traversed in the first list
        struct node* dnext1 = current1->next;

        // next node to be traversed in the first list
        struct node* dnext2 = current2->next;
        current1->next = current2;
        current2->next = dnext1;
        current1 = dnext1;
        current2 = dnext2;
    }
}

// Function to insert node after the end
void insertNode(struct node** start, int val)
{

    // allocate memory
    struct node* temp = (struct node*)malloc(sizeof(struct node));
    temp->data = val;

    // if first node
    if (*start == NULL)
        *start = temp;
    else {

        // move to the end pointer of node
        struct node* dstart = *start;
        while (dstart->next != NULL)
            dstart = dstart->next;
        dstart->next = temp;
    }
}

// function to print the linked list
void display(struct node** start)
{
    struct node* temp = *start;

    // traverse till the entire linked
    // list is printed
    while (temp->next != NULL) {
        printf("%d->", temp->data);
        temp = temp->next;
    }
    printf("%d\n", temp->data);
}

// Driver Code
int main()
{
    // Odd Length Linked List
    struct node* start = NULL;
    insertNode(&start, 1);
    insertNode(&start, 2);
    insertNode(&start, 3);
    insertNode(&start, 4);
    insertNode(&start, 5);

    printf("Before Modifying: ");
    display(&start);
    modifyLL(start);
    printf("After Modifying: ");
    display(&start);

    // Even Length Linked List
    start = NULL;
    insertNode(&start, 1);
    insertNode(&start, 2);
    insertNode(&start, 3);
    insertNode(&start, 4);
    insertNode(&start, 5);
    insertNode(&start, 6);

    printf("\nBefore Modifying: ");
    display(&start);
    modifyLL(start);
    printf("After Modifying: ");
    display(&start);

    return 0;
}

Java

// Java program to sandwich the
// last part of linked list in
// between the first part of
// the linked list
import java.util.*;

class node
{
    int data;
    node next;
    node(int key)
    {
        data = key;
        next = null;
    }
}

class GFG
{

// Function to reverse
// Linked List
public static node reverseLL(node root)
{

     node prev = null,
     current = root, next;
    while (current!= null)
    {
        next = current.next;
        current.next = prev;
        prev = current;
        current = next;
    }

    // root needs to be
    // updated after reversing
    root = prev;
    return root;
}

// Function to modify
// Linked List
public static void modifyLL( node root)
{
    // Find the mid node
     node slow_ptr = root, fast_ptr = root;
    while (fast_ptr != null &&
           fast_ptr.next != null)
    {
        fast_ptr = fast_ptr.next.next;
        slow_ptr = slow_ptr.next;
    }

    // Reverse the second
    // half of the list
    node root2 = reverseLL(slow_ptr.next);

    // partition the list
    slow_ptr.next = null;

     node current1 = root,
          current2 = root2;

    // insert the elements in between
    while (current1 != null &&
           current2 != null)
    {

        // next node to be traversed
        // in the first list
        node dnext1 = current1.next;

        // next node to be traversed
        // in the first list
        node dnext2 = current2.next;
        current1.next = current2;
        current2.next = dnext1;
        current1 = dnext1;
        current2 = dnext2;
    }
}

// Function to insert
// node after the end
public static node insertNode(node start,
                              int val)
{

    // allocate memory
    node temp = new node(val);

    // if first node
    if (start == null)
        start = temp;
    else
    {

        // move to the end
        // pointer of node
         node dstart = start;
        while (dstart.next != null)
            dstart = dstart.next;
        dstart.next = temp;
    }

    return start;
}

// function to print
// the linked list
public static void display(node start)
{
     node temp = start;

    // traverse till the
    // entire linked
    // list is printed
    while (temp.next != null) {
        System.out.print(temp.data + "->");
        temp = temp.next;
    }
    System.out.println(temp.data);
}

// Driver Code
public static void main(String args[])
{
    // Odd Length Linked List
    node start = null;
    start = insertNode(start, 1);
    start = insertNode(start, 2);
    start = insertNode(start, 3);
    start = insertNode(start, 4);
    start = insertNode(start, 5);

    System.out.print("Before Modifying: ");
    display(start);
    modifyLL(start);
    System.out.print("After Modifying: ");
    display(start);

    // Even Length Linked List
    start = null;
    start = insertNode(start, 1);
    start = insertNode(start, 2);
    start = insertNode(start, 3);
    start = insertNode(start, 4);
    start = insertNode(start, 5);
    start = insertNode(start, 6);


    System.out.print("Before Modifying: ");
    display(start);
    modifyLL(start);
    System.out.print("After Modifying: ");
    display(start);
}
}

Python3

# Python3 program to sandwich the last part of
# linked list in between the first part of
# the linked list
class node:
    def __init__(self):
        self.data = 0
        self.next = None

# Function to reverse Linked List
def reverseLL(root):
    prev = None
    current = root
    while (current):
        next = current.next;
        current.next = prev;
        prev = current;
        current = next;

    # root needs to be updated after reversing
    root = prev;
    return root;

# Function to modify Linked List
def modifyLL(root):

    # Find the mid node
    slow_ptr = root
    fast_ptr = root;
    while (fast_ptr and fast_ptr.next):
        fast_ptr = fast_ptr.next.next;
        slow_ptr = slow_ptr.next;

    # Reverse the second half of the list
    root2 = reverseLL(slow_ptr.next);

    # partition the list
    slow_ptr.next = None;
    current1 = root
    current2 = root2;

    # insert the elements in between
    while (current1 and current2):

        # next node to be traversed in the first list
        dnext1 = current1.next;

        # next node to be traversed in the first list
        dnext2 = current2.next;
        current1.next = current2;
        current2.next = dnext1;
        current1 = dnext1;
        current2 = dnext2;

# Function to insert node after the end
def insertNode(start, val):

    # allocate memory
    temp = node()
    temp.data = val;

    # if first node
    if (start == None):
        start = temp;
    else:

        # move to the end pointer of node
        dstart = start;
        while (dstart.next != None):
            dstart = dstart.next;
        dstart.next = temp;
    return start

# function to print the linked list
def display(start):
    temp = start;

    # traverse till the entire linked
    # list is printed
    while (temp.next != None):
        print("{}->".format(temp.data), end = '')
        temp = temp.next;
    print("{}".format(temp.data))

# Driver Code
if __name__=='__main__':

    # Odd Length Linked List
    start = None;
    start = insertNode(start, 1);
    start = insertNode(start, 2);
    start = insertNode(start, 3);
    start = insertNode(start, 4);
    start = insertNode(start, 5);

    print("Before Modifying: ", end = '');
    display(start);
    modifyLL(start);
    print("After Modifying: ", end = '');
    display(start);

    # Even Length Linked List
    start = None;
    start = insertNode(start, 1);
    start = insertNode(start, 2);
    start = insertNode(start, 3);
    start = insertNode(start, 4);
    start = insertNode(start, 5);
    start = insertNode(start, 6);

    print("\nBefore Modifying: ", end = '');
    display(start);
    modifyLL(start);
    print("After Modifying: ", end = '')
    display(start);

# This code is contributed by rutvik_56

C#

// C# program to sandwich the last part
// of linked list in between the first
// part of the linked list
using System;

public class node
{
    public int data;
    public node next;
    public node(int key)
    {
        data = key;
        next = null;
    }
}

public class GFG
{

// Function to reverse
// Linked List
public static node reverseLL(node root)
{
    node prev = null,
    current = root, next;
    while (current!= null)
    {
        next = current.next;
        current.next = prev;
        prev = current;
        current = next;
    }

    // root needs to be
    // updated after reversing
    root = prev;
    return root;
}

// Function to modify
// Linked List
public static void modifyLL( node root)
{
    // Find the mid node
    node slow_ptr = root, fast_ptr = root;
    while (fast_ptr != null &&
           fast_ptr.next != null)
    {
        fast_ptr = fast_ptr.next.next;
        slow_ptr = slow_ptr.next;
    }

    // Reverse the second
    // half of the list
    node root2 = reverseLL(slow_ptr.next);

    // partition the list
    slow_ptr.next = null;

    node current1 = root,
        current2 = root2;

    // insert the elements in between
    while (current1 != null &&
           current2 != null)
    {

        // next node to be traversed
        // in the first list
        node dnext1 = current1.next;

        // next node to be traversed
        // in the first list
        node dnext2 = current2.next;
        current1.next = current2;
        current2.next = dnext1;
        current1 = dnext1;
        current2 = dnext2;
    }
}

// Function to insert
// node after the end
public static node insertNode(node start,
                               int val)
{

    // allocate memory
    node temp = new node(val);

    // if first node
    if (start == null)
        start = temp;
    else
    {

        // move to the end
        // pointer of node
        node dstart = start;
        while (dstart.next != null)
            dstart = dstart.next;
        dstart.next = temp;
    }

    return start;
}

// function to print
// the linked list
public static void display(node start)
{
    node temp = start;

    // traverse till the
    // entire linked
    // list is printed
    while (temp.next != null)
    {
        Console.Write(temp.data + "->");
        temp = temp.next;
    }
    Console.WriteLine(temp.data);
}

// Driver Code
public static void Main(String []args)
{
    // Odd Length Linked List
    node start = null;
    start = insertNode(start, 1);
    start = insertNode(start, 2);
    start = insertNode(start, 3);
    start = insertNode(start, 4);
    start = insertNode(start, 5);

    Console.Write("Before Modifying: ");
    display(start);
    modifyLL(start);
    Console.Write("After Modifying: ");
    display(start);

    // Even Length Linked List
    start = null;
    start = insertNode(start, 1);
    start = insertNode(start, 2);
    start = insertNode(start, 3);
    start = insertNode(start, 4);
    start = insertNode(start, 5);
    start = insertNode(start, 6);

    Console.Write("Before Modifying: ");
    display(start);
    modifyLL(start);
    Console.Write("After Modifying: ");
    display(start);
}
}

// This code has been contributed
// by 29AjayKumar

Javascript

<script>
      // JavaScript program to sandwich the last part
      // of linked list in between the first
      // part of the linked list
      class node {
        constructor(key) {
          this.data = key;
          this.next = null;
        }
      }

      // Function to reverse
      // Linked List
      function reverseLL(root) {
        var prev = null,
          current = root,
          next;
        while (current != null) {
          next = current.next;
          current.next = prev;
          prev = current;
          current = next;
        }

        // root needs to be
        // updated after reversing
        root = prev;
        return root;
      }

      // Function to modify
      // Linked List
      function modifyLL(root)
      {

        // Find the mid node
        var slow_ptr = root,
          fast_ptr = root;
        while (fast_ptr != null && fast_ptr.next != null)
        {
          fast_ptr = fast_ptr.next.next;
          slow_ptr = slow_ptr.next;
        }

        // Reverse the second
        // half of the list
        var root2 = reverseLL(slow_ptr.next);

        // partition the list
        slow_ptr.next = null;

        var current1 = root,
          current2 = root2;

        // insert the elements in between
        while (current1 != null && current2 != null)
        {

          // next node to be traversed
          // in the first list
          var dnext1 = current1.next;

          // next node to be traversed
          // in the first list
          var dnext2 = current2.next;
          current1.next = current2;
          current2.next = dnext1;
          current1 = dnext1;
          current2 = dnext2;
        }
      }

      // Function to insert
      // node after the end
      function insertNode(start, val)
      {

        // allocate memory
        var temp = new node(val);

        // if first node
        if (start == null) start = temp;
        else
        {

          // move to the end
          // pointer of node
          var dstart = start;
          while (dstart.next != null) dstart = dstart.next;
          dstart.next = temp;
        }

        return start;
      }

      // function to print
      // the linked list
      function display(start) {
        var temp = start;

        // traverse till the
        // entire linked
        // list is printed
        while (temp.next != null) {
          document.write(temp.data + "->");
          temp = temp.next;
        }
        document.write(temp.data + "<br>");
      }

      // Driver Code
      // Odd Length Linked List
      var start = null;
      start = insertNode(start, 1);
      start = insertNode(start, 2);
      start = insertNode(start, 3);
      start = insertNode(start, 4);
      start = insertNode(start, 5);

      document.write("Before Modifying: ");
      display(start);
      modifyLL(start);
      document.write("After Modifying: ");
      display(start);

      // Even Length Linked List
      start = null;
      start = insertNode(start, 1);
      start = insertNode(start, 2);
      start = insertNode(start, 3);
      start = insertNode(start, 4);
      start = insertNode(start, 5);
      start = insertNode(start, 6);

      document.write("<br>Before Modifying: ");
      display(start);
      modifyLL(start);
      document.write("After Modifying: ");
      display(start);

      // This code is contributed by rdtank.
    </script>

Output

Before Modifying: 1->2->3->4->5
After Modifying: 1->5->2->4->3

Before Modifying: 1->2->3->4->5->6
After Modifying: 1->6->2->5->3->4

Time Complexity: O(N) 

Exercise: Solve the question without reversing the Linked List from the middle node.