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Merge two unsorted linked lists to get a sorted list – Set 2

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  • Difficulty Level : Easy
  • Last Updated : 30 Nov, 2021
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Given two unsorted Linked Lists, L1 of N nodes and L2 of M nodes, the task is to merge them to get a sorted singly linked list.

Examples:

Input: L1 = 3→5→1, L2 = 6→2→4→9
Output: 1→2→3→4→5→6→9

Input: L1 = 1→5→2, L2 = 3→7
Output: 1→2→3→5→7

Note: A Memory Efficient approach that solves this problem in O(1) Auxiliary Space has been approached here,

Approach: The idea is to use an auxiliary array to store the elements of both the linked lists and then sort the array in increasing order. And finally, insert all the elements back into the linked list. Follow the steps below to solve the problem:

Below is an implementation of the above approach:

C++




// C++ program for the above approach
#include <bits/stdc++.h>
using namespace std;
 
// Linked list node
class Node {
public:
    int data;
    Node* next;
};
 
// Utility function to append key at
// end of linked list
void insertNode(Node** head, int x)
{
    Node* ptr = new Node;
    ptr->data = x;
    ptr->next = NULL;
    if (*head == NULL) {
        *head = ptr;
    }
    else {
        Node* temp;
        temp = *head;
        while (temp->next != NULL) {
            temp = temp->next;
        }
        temp->next = ptr;
    }
}
 
// Utility function to print linkedlist
void display(Node** head)
{
    Node* temp;
    temp = *head;
    if (temp == NULL) {
        cout << "NULL \n";
    }
    else {
        while (temp != NULL) {
            cout << temp->data;
            if (temp->next != NULL)
                cout << "->";
            temp = temp->next;
        }
    }
}
 
// Function to merge two linked lists
void MergeLinkedlist(Node** head1, Node** head2)
{
    Node* ptr;
    ptr = *head1;
    while (ptr->next != NULL) {
        ptr = ptr->next;
    }
 
    // Join linked list by placing address of
    // first node of L2 in the last node of L1
    ptr->next = *head2;
}
 
// Function to merge two unsorted linked
// lists to get a sorted list
void sortLinkedList(Node** head, Node** head1)
{
    // Function call to merge the two lists
    MergeLinkedlist(head, head1);
 
    // Declare a vector
    vector<int> V;
    Node* ptr = *head;
 
    // Push all elements into vector
    while (ptr != NULL) {
        V.push_back(ptr->data);
        ptr = ptr->next;
    }
 
    // Sort the vector
    sort(V.begin(), V.end());
    int index = 0;
    ptr = *head;
 
    // Insert elements in the linked
    // list from the vector
    while (ptr != NULL) {
        ptr->data = V[index];
        index++;
        ptr = ptr->next;
    }
 
    // Display the sorted and
    // merged linked list
    display(head);
}
 
// Driver Code
int main()
{
    // Given linked list, L1
    Node* head1 = NULL;
    insertNode(&head1, 3);
    insertNode(&head1, 5);
    insertNode(&head1, 1);
 
    // Given linked list, L2
    Node* head2 = NULL;
    insertNode(&head2, 6);
    insertNode(&head2, 2);
    insertNode(&head2, 4);
    insertNode(&head2, 9);
 
    // Function Call
    sortLinkedList(&head1, &head2);
 
    return 0;
}

Java




// Java program for the above approach
import java.util.*;
 
class GFG {
 
    // Linked list node
    static class Node {
        int data;
        Node next;
    };
    static Node head1, head2;
   
    // Utility function to append key at
    // end of linked list
    static Node insertNode(Node head, int x)
    {
        Node ptr = new Node();
        ptr.data = x;
        ptr.next = null;
        if (head == null) {
            head = ptr;
        }
        else {
            Node temp;
            temp = head;
            while (temp.next != null) {
                temp = temp.next;
            }
            temp.next = ptr;
        }
 
        return head;
    }
 
    // Utility function to print linkedlist
    static void display(Node head)
    {
        Node temp;
        temp = head;
        if (temp == null) {
            System.out.print("null \n");
        }
        else {
            while (temp != null) {
                System.out.print(temp.data);
                if (temp.next != null)
                    System.out.print("->");
                temp = temp.next;
            }
        }
    }
 
    // Function to merge two linked lists
    static Node MergeLinkedlist()
    {
        Node ptr;
        ptr = head1;
        while (ptr.next != null) {
            ptr = ptr.next;
        }
 
        // Join linked list by placing address of
        // first node of L2 in the last node of L1
        ptr.next = head2;
 
        return head1;
    }
 
    // Function to merge two unsorted linked
    // lists to get a sorted list
    static void sortLinkedList()
    {
        // Function call to merge the two lists
        Node head = MergeLinkedlist();
 
        // Declare a vector
        Vector<Integer> V = new Vector<>();
        Node ptr = head;
 
        // Push all elements into vector
        while (ptr != null) {
            V.add(ptr.data);
            ptr = ptr.next;
        }
        Collections.sort(V);
        // Sort the vector
        ;
        int index = 0;
        ptr = head;
 
        // Insert elements in the linked
        // list from the vector
        while (ptr != null) {
            ptr.data = V.get(index);
            index++;
            ptr = ptr.next;
        }
 
        // Display the sorted and
        // merged linked list
        display(head);
    }
 
    // Driver Code
    public static void main(String[] args)
    {
        // Given linked list, L1
 
        head1 = insertNode(head1, 3);
        head1 = insertNode(head1, 5);
        head1 = insertNode(head1, 1);
 
        // Given linked list, L2
        head2 = null;
        head2 = insertNode(head2, 6);
        head2 = insertNode(head2, 2);
        head2 = insertNode(head2, 4);
        head2 = insertNode(head2, 9);
 
        // Function Call
        sortLinkedList();
    }
}
 
// This code is contributed by umadevi9616

Python3




# Py program for the above approach
 
# Linked list node
class Node:
    def __init__(self, d):
        self.data = d
        self.next = None
 
# Utility function to append key at
# end of linked list
def insertNode(head, x):
    ptr = Node(x)
    if (head == None):
        head = ptr
    else:
        temp = head
        while (temp.next != None):
            temp = temp.next
        temp.next = ptr
    return head
 
# Utility function to print linkedlist
def display(head):
    temp = head
    if (temp == None):
        print ("None")
    else:
        while (temp.next != None):
            print (temp.data,end="->")
            if (temp.next != None):
                print ("",end="")
            temp = temp.next
        print(temp.data)
 
# Function to merge two linked lists
def MergeLinkedlist(head1, head2):
    ptr = head1
    while (ptr.next != None):
        ptr = ptr.next
 
    # Join linked list by placing address of
    # first node of L2 in the last node of L1
    ptr.next = head2
 
    return head1
 
# Function to merge two unsorted linked
# lists to get a sorted list
def sortLinkedList(head1, head2):
    # Function call to merge the two lists
    head1 = MergeLinkedlist(head1, head2)
 
    # Declare a vector
    V = []
    ptr = head1
 
    # Push all elements into vector
    while (ptr != None):
        V.append(ptr.data)
        ptr = ptr.next
 
    # Sort the vector
    V = sorted(V)
    index = 0
    ptr = head1
 
    # Insert elements in the linked
    # list from the vector
    while (ptr != None):
        ptr.data = V[index]
        index += 1
        ptr = ptr.next
 
    # Display the sorted and
    # merged linked list
    display(head1)
 
# Driver Code
if __name__ == '__main__':
    # Given linked list, L1
    head1 = None
    head1 = insertNode(head1, 3)
    head1 = insertNode(head1, 5)
    head1 = insertNode(head1, 1)
 
 
    # Given linked list, L2
    head2 = None
    head2 = insertNode(head2, 6)
    head2 = insertNode(head2, 2)
    head2 = insertNode(head2, 4)
    head2 = insertNode(head2, 9)
 
    # Function Call
    sortLinkedList(head1, head2)
 
# This code is contributed by mohit kumar 29.

C#




// C# program for the above approach
using System;
using System.Collections.Generic;
 
public class GFG {
 
    // Linked list node
public    class Node {
    public    int data;
    public    Node next;
    };
 
    static Node head1, head2;
 
    // Utility function to append key at
    // end of linked list
    static Node insertNode(Node head, int x) {
        Node ptr = new Node();
        ptr.data = x;
        ptr.next = null;
        if (head == null) {
            head = ptr;
        } else {
            Node temp;
            temp = head;
            while (temp.next != null) {
                temp = temp.next;
            }
            temp.next = ptr;
        }
 
        return head;
    }
 
    // Utility function to print linkedlist
    static void display(Node head) {
        Node temp;
        temp = head;
        if (temp == null) {
            Console.Write("null \n");
        } else {
            while (temp != null) {
                Console.Write(temp.data);
                if (temp.next != null)
                    Console.Write("->");
                temp = temp.next;
            }
        }
    }
 
    // Function to merge two linked lists
    static Node MergeLinkedlist() {
        Node ptr;
        ptr = head1;
        while (ptr.next != null) {
            ptr = ptr.next;
        }
 
        // Join linked list by placing address of
        // first node of L2 in the last node of L1
        ptr.next = head2;
 
        return head1;
    }
 
    // Function to merge two unsorted linked
    // lists to get a sorted list
    static void sortList()
    {
       
        // Function call to merge the two lists
        Node head = MergeLinkedlist();
 
        // Declare a vector
        List<int> V = new List<int>();
        Node ptr = head;
 
        // Push all elements into vector
        while (ptr != null) {
            V.Add(ptr.data);
            ptr = ptr.next;
        }
        V.Sort();
        // Sort the vector
        ;
        int index = 0;
        ptr = head;
 
        // Insert elements in the linked
        // list from the vector
        while (ptr != null) {
            ptr.data = V[index];
            index++;
            ptr = ptr.next;
        }
 
        // Display the sorted and
        // merged linked list
        display(head);
    }
 
    // Driver Code
    public static void Main(String[] args)
    {
       
        // Given linked list, L1
 
        head1 = insertNode(head1, 3);
        head1 = insertNode(head1, 5);
        head1 = insertNode(head1, 1);
 
        // Given linked list, L2
        head2 = null;
        head2 = insertNode(head2, 6);
        head2 = insertNode(head2, 2);
        head2 = insertNode(head2, 4);
        head2 = insertNode(head2, 9);
 
        // Function Call
        sortList();
    }
}
 
// This code is contributed by umadevi9616

Javascript




// Javascript program for the above approach
 
// Linked list node
class Node {
    constructor(d) {
        this.data = d
        this.next = null
    }
}
 
// Utility function to append key at
// end of linked list
function insertNode(head, x) {
    ptr = new Node(x)
    if (head == null) {
        head = ptr
    } else {
        temp = head
        while (temp.next != null) {
            temp = temp.next
        }
        temp.next = ptr
    }
    return head
}
 
// Utility function to print linkedlist
function display(head) {
    let temp = head
    if (temp == null) {
        document.write("null")
    }
    else {
        while (temp.next != null) {
            document.write(temp.data, end = "->")
            if (temp.next != null)
                document.write("", end = "")
            temp = temp.next
        }
        document.write(temp.data)
    }
}
 
// Function to merge two linked lists
function MergeLinkedlist(head1, head2) {
    let ptr = head1
    while (ptr.next != null)
        ptr = ptr.next
 
    // Join linked list by placing address of
    // first node of L2 in the last node of L1
    ptr.next = head2
 
    return head1
}
 
// Function to merge two unsorted linked
// lists to get a sorted list
function sortLinkedList(head1, head2) {
    // Function call to merge the two lists
    head1 = MergeLinkedlist(head1, head2)
 
    // Declare a vector
    let V = []
    let ptr = head1
 
    // Push all elements into vector
    while (ptr != null) {
        V.push(ptr.data)
        ptr = ptr.next
    }
 
    // Sort the vector
    V = V.sort((a, b) => a - b)
    index = 0
    ptr = head1
 
    // Insert elements in the linked
    // list from the vector
    while (ptr != null) {
        ptr.data = V[index]
        index += 1
        ptr = ptr.next
    }
 
    // Display the sorted and
    // merged linked list
    display(head1)
}
 
// Driver Code
 
// Given linked list, L1
let head1 = null;
head1 = insertNode(head1, 3)
head1 = insertNode(head1, 5)
head1 = insertNode(head1, 1)
 
 
// Given linked list, L2
let head2 = null
head2 = insertNode(head2, 6)
head2 = insertNode(head2, 2)
head2 = insertNode(head2, 4)
head2 = insertNode(head2, 9)
 
// Function Call
sortLinkedList(head1, head2)
 
// This code is contributed by gfgking

Output

1->2->3->4->5->6->9

Time Complexity: O((N+M)*log(N+M))
Auxiliary Space: O(N+M)


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