Merge two unsorted linked lists to get a sorted list

Given two unsorted Linked List, the task is to merge them to get a sorted singly linked list.
Examples:

Input: List 1 = 3 -> 1 -> 5, List 2 = 6-> 2 -> 4 
Output: 1 -> 2 -> 3 -> 4 -> 5 -> 6

Input: List 1 = 4 -> 7 -> 5, List 2 = 2-> 1 -> 8 -> 1 
Output: 1 -> 1 -> 2 -> 4 -> 5 -> 7 -> 8 

Naive Approach: The naive approach is to sort the given linked lists and then merge the two sorted linked lists together into one list in increasing order.
To solve the problem mentioned above the naive method is to sort the two linked lists individually and merge the two linked lists together into one list which is in increasing order.

Efficient Approach: To optimize the above method we will concatenate the two linked lists and then sort it using any sorting algorithm. Below are the steps: 

1. Concatenate the two lists by traversing the first list until we reach it’s a tail node and then point the next of the tail node to the head node of the second list. Store this concatenated list in the first list.
2. Sort the above-merged linked list. Here, we will use a bubble sort. So, if node->next->data is less then node->data, then swap the data of the two adjacent nodes.

Below is the implementation of the above approach:

C++

// C++ program for the above approach
#include <bits/stdc++.h>
using namespace std;

// Create structure for a node
struct node {
    int data;
    node* next;
};

// Function to print the linked list
void setData(node* head)
{
    node* tmp;

    // Store the head of the linked
    // list into a temporary node*
    // and iterate
    tmp = head;

    while (tmp != NULL) {

        cout << tmp->data
             << " -> ";
        tmp = tmp->next;
    }
}

// Function takes the head of the
// LinkedList and the data as
// argument and if no LinkedList
// exists, it creates one with the
// head pointing to first node.
// If it exists already, it appends
// given node at end of the last node
node* getData(node* head, int num)
{

    // Create a new node
    node* temp = new node;
    node* tail = head;

    // Insert data into the temporary
    // node and point it's next to NULL
    temp->data = num;
    temp->next = NULL;

    // Check if head is null, create a
    // linked list with temp as head
    // and tail of the list
    if (head == NULL) {
        head = temp;
        tail = temp;
    }

    // Else insert the temporary node
    // after the tail of the existing
    // node and make the temporary node
    // as the tail of the linked list
    else {

        while (tail != NULL) {

            if (tail->next == NULL) {
                tail->next = temp;
                tail = tail->next;
            }
            tail = tail->next;
        }
    }

    // Return the list
    return head;
}

// Function to concatenate the two lists
node* mergelists(node** head1,
                 node** head2)
{

    node* tail = *head1;

    // Iterate through the head1 to find the
    // last node join the next of last node
    // of head1 to the 1st node of head2
    while (tail != NULL) {

        if (tail->next == NULL
            && head2 != NULL) {
            tail->next = *head2;
            break;
        }
        tail = tail->next;
    }

    // return the concatenated lists as a
    // single list - head1
    return *head1;
}

// Sort the linked list using bubble sort
void sortlist(node** head1)
{
    node* curr = *head1;
    node* temp = *head1;

    // Compares two adjacent elements
    // and swaps if the first element
    // is greater than the other one.
    while (curr->next != NULL) {

        temp = curr->next;
        while (temp != NULL) {

            if (temp->data < curr->data) {
                int t = temp->data;
                temp->data = curr->data;
                curr->data = t;
            }
            temp = temp->next;
        }
        curr = curr->next;
    }
}

// Driver Code
int main()
{
    node* head1 = new node;
    node* head2 = new node;

    head1 = NULL;
    head2 = NULL;

    // Given Linked List 1
    head1 = getData(head1, 4);
    head1 = getData(head1, 7);
    head1 = getData(head1, 5);

    // Given Linked List 2
    head2 = getData(head2, 2);
    head2 = getData(head2, 1);
    head2 = getData(head2, 8);
    head2 = getData(head2, 1);

    // Merge the two lists
    // in a single list
    head1 = mergelists(&head1,
                       &head2);

    // Sort the unsorted merged list
    sortlist(&head1);

    // Print the final
    // sorted merged list
    setData(head1);
    return 0;
}

Java

// Java program for
// the above approach
class GFG{

static node head1 = null;
static node head2 = null;

// Create structure for a node
static class node
{
  int data;
  node next;
};

// Function to print
// the linked list
static void setData(node head)
{
  node tmp;

  // Store the head of the linked
  // list into a temporary node
  // and iterate
  tmp = head;

  while (tmp != null)
  {
    System.out.print(tmp.data + " -> ");
    tmp = tmp.next;
  }
}

// Function takes the head of the
// LinkedList and the data as
// argument and if no LinkedList
// exists, it creates one with the
// head pointing to first node.
// If it exists already, it appends
// given node at end of the last node
static node getData(node head, int num)
{
  // Create a new node
  node temp = new node();
  node tail = head;

  // Insert data into the temporary
  // node and point it's next to null
  temp.data = num;
  temp.next = null;

  // Check if head is null, create a
  // linked list with temp as head
  // and tail of the list
  if (head == null)
  {
    head = temp;
    tail = temp;
  }

  // Else insert the temporary node
  // after the tail of the existing
  // node and make the temporary node
  // as the tail of the linked list
  else
  {
    while (tail != null)
    {
      if (tail.next == null)
      {
        tail.next = temp;
        tail = tail.next;
      }
      tail = tail.next;
    }
  }

  // Return the list
  return head;
}

// Function to concatenate
// the two lists
static node mergelists()
{
  node tail = head1;

  // Iterate through the
  // head1 to find the
  // last node join the
  // next of last node
  // of head1 to the
  // 1st node of head2
  while (tail != null)
  {
    if (tail.next == null &&
        head2 != null)
    {
      tail.next = head2;
      break;
    }
    tail = tail.next;
  }

  // return the concatenated
  // lists as a single list - head1
  return head1;
}

// Sort the linked list
// using bubble sort
static void sortlist()
{
  node curr = head1;
  node temp = head1;

  // Compares two adjacent elements
  // and swaps if the first element
  // is greater than the other one.
  while (curr.next != null)
  {
    temp = curr.next;
    while (temp != null)
    {
      if (temp.data < curr.data)
      {
        int t = temp.data;
        temp.data = curr.data;
        curr.data = t;
      }
      temp = temp.next;
    }
    curr = curr.next;
  }
}

// Driver Code
public static void main(String[] args)
{
  // Given Linked List 1
  head1 = getData(head1, 4);
  head1 = getData(head1, 7);
  head1 = getData(head1, 5);

  // Given Linked List 2
  head2 = getData(head2, 2);
  head2 = getData(head2, 1);
  head2 = getData(head2, 8);
  head2 = getData(head2, 1);

  // Merge the two lists
  // in a single list
  head1 = mergelists();

  // Sort the unsorted merged list
  sortlist();

  // Print the final
  // sorted merged list
  setData(head1);
}
}

// This code is contributed by shikhasingrajput

Python3

# Python3 program for the
# above approach

# Create structure for a node
class node:

    def __init__(self, x):

        self.data = x
        self.next = None

# Function to print the linked
# list
def setData(head):

    # Store the head of the
    # linked list into a
    # temporary node* and
    # iterate
    tmp = head

    while (tmp != None):
        print(tmp.data,
              end = " -> ")
        tmp = tmp.next

# Function takes the head of the
# LinkedList and the data as
# argument and if no LinkedList
# exists, it creates one with the
# head pointing to first node.
# If it exists already, it appends
# given node at end of the last node
def getData(head, num):

    # Create a new node
    temp = node(-1)
    tail = head

    # Insert data into the temporary
    # node and point it's next to NULL
    temp.data = num
    temp.next = None

    # Check if head is null, create a
    # linked list with temp as head
    # and tail of the list
    if (head == None):
        head = temp
        tail = temp

    # Else insert the temporary node
    # after the tail of the existing
    # node and make the temporary node
    # as the tail of the linked list
    else:
        while (tail != None):
            if (tail.next == None):
                tail.next = temp
                tail = tail.next
            tail = tail.next

    # Return the list
    return head

# Function to concatenate the
# two lists
def mergelists(head1,head2):

    tail = head1

    # Iterate through the head1 to
    # find the last node join the
    # next of last node of head1
    # to the 1st node of head2
    while (tail != None):
        if (tail.next == None
            and head2 != None):
            tail.next =head2
            break
        tail = tail.next

    # return the concatenated
    # lists as a single list
    # - head1
    return head1

# Sort the linked list using
# bubble sort
def sortlist(head1):

    curr = head1
    temp = head1

    # Compares two adjacent elements
    # and swaps if the first element
    # is greater than the other one.
    while (curr.next != None):
        temp = curr.next
        while (temp != None):
            if (temp.data < curr.data):
                t = temp.data
                temp.data = curr.data
                curr.data = t
            temp = temp.next
        curr = curr.next

# Driver Code
if __name__ == '__main__':

    head1 = node(-1)
    head2 = node(-1)

    head1 = None
    head2 = None

    # Given Linked List 1
    head1 = getData(head1, 4)
    head1 = getData(head1, 7)
    head1 = getData(head1, 5)

    # Given Linked List 2
    head2 = getData(head2, 2)
    head2 = getData(head2, 1)
    head2 = getData(head2, 8)
    head2 = getData(head2, 1)

    # Merge the two lists
    # in a single list
    head1 = mergelists(head1,head2)

    # Sort the unsorted merged list
    sortlist(head1)

    # Print the final
    # sorted merged list
    setData(head1)

# This code is contributed by Mohit Kumar 29

C#

// C# program for
// the above approach
using System;

class GFG{

static node head1 = null;
static node head2 = null;

// Create structure for a node
class node
{
    public int data;
    public node next;
};

// Function to print
// the linked list
static void setData(node head)
{
    node tmp;

    // Store the head of the linked
    // list into a temporary node
    // and iterate
    tmp = head;

    while (tmp != null)
    {
        Console.Write(tmp.data + " -> ");
        tmp = tmp.next;
    }
}

// Function takes the head of
//the List and the data as
// argument and if no List
// exists, it creates one with the
// head pointing to first node.
// If it exists already, it appends
// given node at end of the last node
static node getData(node head, int num)
{

    // Create a new node
    node temp = new node();
    node tail = head;

    // Insert data into the temporary
    // node and point it's next to null
    temp.data = num;
    temp.next = null;

    // Check if head is null, create a
    // linked list with temp as head
    // and tail of the list
    if (head == null)
    {
        head = temp;
        tail = temp;
    }

    // Else insert the temporary node
    // after the tail of the existing
    // node and make the temporary node
    // as the tail of the linked list
    else
    {
        while (tail != null)
        {
            if (tail.next == null)
            {
                tail.next = temp;
                tail = tail.next;
            }
            tail = tail.next;
        }
    }

    // Return the list
    return head;
}

// Function to concatenate
// the two lists
static node mergelists()
{
    node tail = head1;

    // Iterate through the
    // head1 to find the
    // last node join the
    // next of last node
    // of head1 to the
    // 1st node of head2
    while (tail != null)
    {
        if (tail.next == null &&
                head2 != null)
        {
            tail.next = head2;
            break;
        }
        tail = tail.next;
    }

    // return the concatenated
    // lists as a single list - head1
    return head1;
}

// Sort the linked list
// using bubble sort
static void sortlist()
{
    node curr = head1;
    node temp = head1;

    // Compares two adjacent elements
    // and swaps if the first element
    // is greater than the other one.
    while (curr.next != null)
    {
        temp = curr.next;
        while (temp != null)
        {
            if (temp.data < curr.data)
            {
                int t = temp.data;
                temp.data = curr.data;
                curr.data = t;
            }
            temp = temp.next;
        }
        curr = curr.next;
    }
}

// Driver Code
public static void Main(String[] args)
{

    // Given Linked List 1
    head1 = getData(head1, 4);
    head1 = getData(head1, 7);
    head1 = getData(head1, 5);

    // Given Linked List 2
    head2 = getData(head2, 2);
    head2 = getData(head2, 1);
    head2 = getData(head2, 8);
    head2 = getData(head2, 1);

    // Merge the two lists
    // in a single list
    head1 = mergelists();

    // Sort the unsorted merged list
    sortlist();

    // Print the final
    // sorted merged list
    setData(head1);
}
}

// This code is contributed by Amit Katiyar

Javascript

<script>
// javascript program for
// the above approach
    var head1 = null;
    var head2 = null;

    // Create structure for a node
    class node {
    constructor(){
         this.data=0;
         this.next = null;
        }
    }

    // Function to print
    // the linked list
    function setData( head) {
        var tmp;

        // Store the head of the linked
        // list into a temporary node
        // and iterate
        tmp = head;

        while (tmp != null) {
            document.write(tmp.data + " -> ");
            tmp = tmp.next;
        }
    }

    // Function takes the head of the
    // LinkedList and the data as
    // argument and if no LinkedList
    // exists, it creates one with the
    // head pointing to first node.
    // If it exists already, it appends
    // given node at end of the last node
    function getData( head , num) {
        // Create a new node
         temp = new node();
        var tail = head;

        // Insert data into the temporary
        // node and point it's next to null
        temp.data = num;
        temp.next = null;

        // Check if head is null, create a
        // linked list with temp as head
        // and tail of the list
        if (head == null) {
            head = temp;
            tail = temp;
        }

        // Else insert the temporary node
        // after the tail of the existing
        // node and make the temporary node
        // as the tail of the linked list
        else {
            while (tail != null) {
                if (tail.next == null) {
                    tail.next = temp;
                    tail = tail.next;
                }
                tail = tail.next;
            }
        }

        // Return the list
        return head;
    }

    // Function to concatenate
    // the two lists
    function mergelists() {
         tail = head1;

        // Iterate through the
        // head1 to find the
        // last node join the
        // next of last node
        // of head1 to the
        // 1st node of head2
        while (tail != null) {
            if (tail.next == null && head2 != null) {
                tail.next = head2;
                break;
            }
            tail = tail.next;
        }

        // return the concatenated
        // lists as a single list - head1
        return head1;
    }

    // Sort the linked list
    // using bubble sort
    function sortlist() {
         curr = head1;
         temp = head1;

        // Compares two adjacent elements
        // and swaps if the first element
        // is greater than the other one.
        while (curr.next != null) {
            temp = curr.next;
            while (temp != null) {
                if (temp.data < curr.data) {
                    var t = temp.data;
                    temp.data = curr.data;
                    curr.data = t;
                }
                temp = temp.next;
            }
            curr = curr.next;
        }
    }

    // Driver Code

        // Given Linked List 1
        head1 = getData(head1, 4);
        head1 = getData(head1, 7);
        head1 = getData(head1, 5);

        // Given Linked List 2
        head2 = getData(head2, 2);
        head2 = getData(head2, 1);
        head2 = getData(head2, 8);
        head2 = getData(head2, 1);

        // Merge the two lists
        // in a single list
        head1 = mergelists();

        // Sort the unsorted merged list
        sortlist();

        // Print the final
        // sorted merged list
        setData(head1);

// This code is contributed by umadevi9616.
</script>

Output:

1 -> 1 -> 2 -> 4 -> 5 -> 7 -> 8

Time Complexity: O((M+N)^2) where M and N are the lengths of the two given linked lists. We are merging the two list and performing bubble sort on the merged list. The time complexity of bubble sort is quadratic.
Auxiliary Space: O(1)