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Difference of two Linked Lists using Merge sort

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Given two Linked List, the task is to create a Linked List to store the difference of Linked List 1 with Linked List 2, i.e. the elements present in List 1 but not in List 2.
Examples: 
 

Input: 
List1: 10 -> 15 -> 4 ->20, 
List2: 8 -> 4 -> 2 -> 10 
Output: 15 -> 20 
Explanation: 
In the given linked list elements 15 and 20 are present in the list 1 but not in list 2.
Input: 
List1: 2 -> 4 -> 8 -> 10, 
List2: 8 -> 10 
Output: 2 -> 4 
Explanation: 
In the given linked list 1 elements 2 and 4 are present in the list 1 but not in list 2. 
 


 


Approach: 
 

  • Sort both Linked Lists using merge sort.
  • Linearly scan both sorted lists to get the difference with two pointers on it p1 and p2 and compare the data of the nodes in the linked list and perform following in below three cases – 
    1. If p1.data == p2.data then, p1.data cannot be in the difference list, So move the pointers p1 and p2 ahead.
    2. If p1.data > p2.data then, p1.data may be present in list 2 in further nodes, So move the pointer p2 ahead.
    3. If p1.data > p2.data then, p1.data cannot be present in list 2 now, So add the data of p1 into difference list and move pointer p1 ahead.
  • If the end of list 2 is reached, insert all the remaining elements of list 1 into the difference list.


Below is the implementation of the above approach.
 

C++

//C++ program to implement above approach
 
#include <iostream>
 
class Node {
public:
    int data;
    Node* next;
 
    Node(int val) : data(val), next(nullptr) {}
};
 
class LinkedList {
public:
    Node* head;
 
    LinkedList() : head(nullptr) {}
 
    // Function to insert a node at the end of the linked list
    void append(int data) {
        Node* temp = new Node(data);
        if (head == nullptr) {
            head = temp;
        } else {
            Node* p = head;
            while (p->next != nullptr) {
                p = p->next;
            }
            p->next = temp;
        }
    }
 
    // Function to find the middle node of the linked list
    Node* get_mid(Node* head) {
        if (head == nullptr) {
            return head;
        }
        Node* slow = head;
        Node* fast = head;
        while (fast->next != nullptr && fast->next->next != nullptr) {
            slow = slow->next;
            fast = fast->next->next;
        }
        return slow;
    }
 
    // Recursive method to merge two halves after sorting
    Node* merge(Node* l, Node* r) {
        if (l == nullptr) return r;
        if (r == nullptr) return l;
 
        Node* result;
        if (l->data <= r->data) {
            result = l;
            result->next = merge(l->next, r);
        } else {
            result = r;
            result->next = merge(l, r->next);
        }
        return result;
    }
 
    // Recursive method to divide the list into two halves until 1 node left
    Node* merge_sort(Node* head) {
        if (head == nullptr || head->next == nullptr) {
            return head;
        }
        Node* mid = get_mid(head);
        Node* next_to_mid = mid->next;
        mid->next = nullptr;
        Node* left = merge_sort(head);
        Node* right = merge_sort(next_to_mid);
        Node* sorted_merge = merge(left, right);
        return sorted_merge;
    }
 
    // Function to print the list elements
    void display() {
        Node* p = head;
        while (p != nullptr) {
            std::cout << p->data << " ";
            p = p->next;
        }
        std::cout << std::endl;
    }
};
 
// Function to get the difference list
LinkedList get_difference(Node* p1, Node* p2) {
    LinkedList difference_list;
    // Scan the lists
    while (p1 != nullptr && p2 != nullptr) {
        // Condition to check if the data of both pointers are the same, then move ahead
        if (p2->data == p1->data) {
            p1 = p1->next;
            p2 = p2->next;
        } else if (p2->data < p1->data) { // Condition to check if the data of the second pointer is smaller than the first, then move the second pointer ahead
            p2 = p2->next;
        } else { // Condition when the data of the first pointer is greater than the second, then append into the difference list and move
            difference_list.append(p1->data);
            p1 = p1->next;
        }
    }
 
    // If the end of list2 is reached, there may be some nodes in List 1 left to be scanned,
    // they all will be inserted in the difference list
    while (p1 != nullptr) {
        difference_list.append(p1->data);
        p1 = p1->next;
    }
 
    return difference_list;
}
 
int main() {
    // Linked List 1
    LinkedList list1;
    list1.append(2);
    list1.append(6);
    list1.append(8);
    list1.append(1);
 
    // Linked List 2
    LinkedList list2;
    list2.append(4);
    list2.append(1);
    list2.append(9);
 
    // Sort both linked lists
    list1.head = list1.merge_sort(list1.head);
    list2.head = list2.merge_sort(list2.head);
 
    // Get the difference list
    LinkedList result = get_difference(list1.head, list2.head);
 
    if (result.head) {
        result.display();
    } else {
        std::cout << "Lists are equal" << std::endl;
    }
 
    return 0;
}

                    

Java

class Node {
    int data;
    Node next;
 
    Node(int data) {
        this.data = data;
        this.next = null;
    }
}
 
class LinkedList {
    Node head;
 
    LinkedList() {
        head = null;
    }
 
    // Function to insert a node at the end of Linked List
    void append(int data) {
        Node newNode = new Node(data);
        if (head == null) {
            head = newNode;
        } else {
            Node current = head;
            while (current.next != null) {
                current = current.next;
            }
            current.next = newNode;
        }
    }
 
    // Function to find the middle node of the Linked List
    Node getMid(Node head) {
        if (head == null) {
            return head;
        }
        Node slow = head;
        Node fast = head;
        while (fast.next != null && fast.next.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        return slow;
    }
 
    // Recursive method to merge two halves after sorting
    Node merge(Node left, Node right) {
        if (left == null) {
            return right;
        }
        if (right == null) {
            return left;
        }
 
        Node result;
        if (left.data <= right.data) {
            result = left;
            result.next = merge(left.next, right);
        } else {
            result = right;
            result.next = merge(left, right.next);
        }
        return result;
    }
 
    // Recursive method to divide the list into two halves until 1 node left
    Node mergeSort(Node head) {
        if (head == null || head.next == null) {
            return head;
        }
 
        Node mid = getMid(head);
        Node nextToMid = mid.next;
        mid.next = null;
        Node left = mergeSort(head);
        Node right = mergeSort(nextToMid);
        return merge(left, right);
    }
 
    // Function to print the list elements
    void display() {
        Node current = head;
        while (current != null) {
            System.out.print(current.data + " ");
            current = current.next;
        }
        System.out.println();
    }
 
    // Function to get the difference list
    static LinkedList getDifference(Node p1, Node p2) {
        LinkedList differenceList = new LinkedList();
 
        // Scan the lists
        while (p1 != null && p2 != null) {
 
            // Condition to check if the data of both pointers are the same, then move ahead
            if (p2.data == p1.data) {
                p1 = p1.next;
                p2 = p2.next;
            } else if (p2.data < p1.data) { // Condition to check if the data of the second pointer is smaller than the first, then move the second pointer ahead
                p2 = p2.next;
            } else { // Condition when the data of the first pointer is greater than the second, then append into the difference list and move
                differenceList.append(p1.data);
                p1 = p1.next;
            }
        }
 
        // If the end of list2 is reached, there may be some nodes in List 1 left to be scanned, they all will be inserted in the difference list
        while (p1 != null) {
            differenceList.append(p1.data);
            p1 = p1.next;
        }
 
        return differenceList;
    }
}
 
public class Main {
    public static void main(String[] args) {
        // Linked List 1
        LinkedList list1 = new LinkedList();
        list1.append(2);
        list1.append(6);
        list1.append(8);
        list1.append(1);
 
        // Linked List 2
        LinkedList list2 = new LinkedList();
        list2.append(4);
        list2.append(1);
        list2.append(9);
 
        // Sort both the linked lists
        list1.head = list1.mergeSort(list1.head);
        list2.head = list2.mergeSort(list2.head);
 
        // Get the difference list
        LinkedList result = LinkedList.getDifference(list1.head, list2.head);
 
        if (result.head != null) {
            result.display();
        } else {
            System.out.println("Lists are equal");
        }
    }
}

                    

Python3

# Python implementation to create
# a difference Linked List of
# two Linked Lists
 
# Node of the Linked List
class Node:
    def __init__(self, data):
        self.data = data
        self.next = None
 
# Linked List
class linked_list:
    def __init__(self):
        self.head = None
     
    # Function to insert a node
    # at the end of Linked List
    def append(self, data):
        temp = Node(data)
        if self.head == None:
            self.head = temp
        else:
            p = self.head
            while p.next != None:
                p = p.next
            p.next = temp
 
    # Function to find the middle
    # node of the Linked List
    def get_mid(self, head):
        if head == None:
            return head
        slow = fast = head
        while fast.next != None \
           and fast.next.next != None:
            slow = slow.next
            fast = fast.next.next
        return slow
 
    # Recursive method to merge the
    # two half after sorting
    def merge(self, l, r):
        if l == None:return r
        if r == None:return l
 
        if l.data<= r.data:
            result = l
            result.next = \
                self.merge(l.next, r)
        else:
            result = r
            result.next = \
                self.merge(l, r.next)
        return result
 
    # Recursive method to divide the
    # list into two half until 1 node left
    def merge_sort(self, head):
        if head == None or head.next == None:
            return head
        mid = self.get_mid(head)
        next_to_mid = mid.next
        mid.next = None
        left = self.merge_sort(head)
        right = self.merge_sort(next_to_mid)
        sorted_merge = self.merge(left, right)
        return sorted_merge
 
    # Function to print the list elements
    def display(self):
        p = self.head
        while p != None:
            print(p.data, end =' ')
            p = p.next
        print()
 
# Function to get the difference list
def get_difference(p1, p2):
    difference_list = linked_list()
    # Scan the lists
    while p1 != None and p2 != None:
         
        # Condition to check if the
        # Data of the both pointer are
        # same then move ahead
        if p2.data == p1.data:
            p1 = p1.next
            p2 = p2.next
         
        # Condition to check if the
        # Data of the first pointer is
        # greater than second then
        # move second pointer ahead
        elif p2.data<p1.data:
            p2 = p2.next
         
        # Condition when first pointer
        # data is greater than the
        # second pointer then append
        # into the difference list and move
        else:
            difference_list.append(p1.data)
            p1 = p1.next
 
    # If end of list2 is reached,
    # there may be some nodes in
    # List 1 left to be scanned,
    # they all will be inserted
    # in the difference list
    if p2 == None:
        while p1:
            difference_list.append(p1.data)
            p1 = p1.next
 
    return difference_list
 
# Driver Code
if __name__ == '__main__':
    # Linked List 1
    list1 = linked_list()
    list1.append(2)
    list1.append(6)
    list1.append(8)
    list1.append(1)
     
    # Linked List 2
    list2 = linked_list()
    list2.append(4)
    list2.append(1)
    list2.append(9)
     
    # Sort both the linkedlists
    list1.head = list1.merge_sort(
                   list1.head
                 )
    list2.head = list2.merge_sort(
                   list2.head
                 )
     
    # Get difference list
    result = get_difference(
               list1.head, list2.head
             )
    if result.head:
        result.display()
     
    # if difference list is empty,
    # then lists are equal
    else:
        print('Lists are equal')

                    

C#

using System;
class Node {
    public int data;
    public Node next;
 
    public Node(int data)
    {
        this.data = data;
        this.next = null;
    }
}
 
class LinkedList {
    public Node head;
 
    public LinkedList() { head = null; }
 
    // Function to insert a node at the end of Linked List
    public void Append(int data)
    {
        Node newNode = new Node(data);
        if (head == null) {
            head = newNode;
        }
        else {
            Node current = head;
            while (current.next != null) {
                current = current.next;
            }
            current.next = newNode;
        }
    }
 
    // Function to find the middle node of the Linked List
    public Node GetMid(Node head)
    {
        if (head == null) {
            return head;
        }
        Node slow = head;
        Node fast = head;
        while (fast.next != null
               && fast.next.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        return slow;
    }
 
    // Recursive method to merge two halves after sorting
    public Node Merge(Node left, Node right)
    {
        if (left == null) {
            return right;
        }
        if (right == null) {
            return left;
        }
 
        Node result;
        if (left.data <= right.data) {
            result = left;
            result.next = Merge(left.next, right);
        }
        else {
            result = right;
            result.next = Merge(left, right.next);
        }
        return result;
    }
 
    // Recursive method to divide the list into two halves
    // until 1 node left
    public Node MergeSort(Node head)
    {
        if (head == null || head.next == null) {
            return head;
        }
 
        Node mid = GetMid(head);
        Node nextToMid = mid.next;
        mid.next = null;
        Node left = MergeSort(head);
        Node right = MergeSort(nextToMid);
        return Merge(left, right);
    }
 
    // Function to print the list elements
    public void Display()
    {
        Node current = head;
        while (current != null) {
            Console.Write(current.data + " ");
            current = current.next;
        }
        Console.WriteLine();
    }
 
    // Function to get the difference list
    public static LinkedList GetDifference(Node p1, Node p2)
    {
        LinkedList differenceList = new LinkedList();
 
        // Scan the lists
        while (p1 != null && p2 != null) {
            // Condition to check if the data of both
            // pointers are the same, then move ahead
            if (p2.data == p1.data) {
                p1 = p1.next;
                p2 = p2.next;
            }
            else if (p2.data
                     < p1.data) // Condition to check if the
                                // data of the second
                                // pointer is smaller than
                                // the first, then move the
                                // second pointer ahead
            {
                p2 = p2.next;
            }
            else { // Condition when the data of the first
                   // pointer is greater than the second,
                   // then append into the difference list
                   // and move
                differenceList.Append(p1.data);
                p1 = p1.next;
            }
        }
 
        // If the end of list2 is reached, there may be some
        // nodes in List 1 left to be scanned, they all will
        // be inserted in the difference list
        while (p1 != null) {
            differenceList.Append(p1.data);
            p1 = p1.next;
        }
 
        return differenceList;
    }
}
 
class Program {
    public static void Main(string[] args)
    {
        // Linked List 1
        LinkedList list1 = new LinkedList();
        list1.Append(2);
        list1.Append(6);
        list1.Append(8);
        list1.Append(1);
 
        // Linked List 2
        LinkedList list2 = new LinkedList();
        list2.Append(4);
        list2.Append(1);
        list2.Append(9);
 
        // Sort both the linked lists
        list1.head = list1.MergeSort(list1.head);
        list2.head = list2.MergeSort(list2.head);
 
        // Get the difference list
        LinkedList result = LinkedList.GetDifference(
            list1.head, list2.head);
 
        if (result.head != null) {
            result.Display();
        }
        else {
            Console.WriteLine("Lists are equal");
        }
    }
}

                    

Javascript

class Node {
  constructor(data) {
    this.data = data;
    this.next = null;
  }
}
 
class LinkedList {
  constructor() {
    this.head = null;
  }
 
  // Function to insert a node at the end of Linked List
  append(data) {
    const newNode = new Node(data);
    if (this.head === null) {
      this.head = newNode;
    } else {
      let current = this.head;
      while (current.next !== null) {
        current = current.next;
      }
      current.next = newNode;
    }
  }
 
  // Function to find the middle node of the Linked List
  getMid(head) {
    if (head === null) {
      return head;
    }
    let slow = head;
    let fast = head;
    while (fast.next !== null && fast.next.next !== null) {
      slow = slow.next;
      fast = fast.next.next;
    }
    return slow;
  }
 
  // Recursive method to merge two halves after sorting
  merge(left, right) {
    if (left === null) {
      return right;
    }
    if (right === null) {
      return left;
    }
 
    let result;
    if (left.data <= right.data) {
      result = left;
      result.next = this.merge(left.next, right);
    } else {
      result = right;
      result.next = this.merge(left, right.next);
    }
    return result;
  }
 
  // Recursive method to divide the list into two halves until 1 node left
  mergeSort(head) {
    if (head === null || head.next === null) {
      return head;
    }
 
    const mid = this.getMid(head);
    const nextToMid = mid.next;
    mid.next = null;
    const left = this.mergeSort(head);
    const right = this.mergeSort(nextToMid);
    return this.merge(left, right);
  }
 
  // Function to print the list elements
  display() {
    let current = this.head;
    let result = '';
    while (current !== null) {
      result += current.data + ' ';
      current = current.next;
    }
    console.log(result.trim());
  }
 
  // Function to get the difference list
  static getDifference(p1, p2) {
    const differenceList = new LinkedList();
 
    // Scan the lists
    while (p1 !== null && p2 !== null) {
      // Condition to check if the data of both pointers are the same, then move ahead
      if (p2.data === p1.data) {
        p1 = p1.next;
        p2 = p2.next;
      } else if (p2.data < p1.data) {
        // Condition to check if the data of the second pointer is smaller than the first, then move the second pointer ahead
        p2 = p2.next;
      } else {
        // Condition when the data of the first pointer is greater than the second, then append into the difference list and move
        differenceList.append(p1.data);
        p1 = p1.next;
      }
    }
 
    // If the end of list2 is reached, there may be some nodes in List 1 left to be scanned, they all will be inserted in the difference list
    while (p1 !== null) {
      differenceList.append(p1.data);
      p1 = p1.next;
    }
 
    return differenceList;
  }
}
 
// Linked List 1
const list1 = new LinkedList();
list1.append(2);
list1.append(6);
list1.append(8);
list1.append(1);
 
// Linked List 2
const list2 = new LinkedList();
list2.append(4);
list2.append(1);
list2.append(9);
 
// Sort both the linked lists
list1.head = list1.mergeSort(list1.head);
list2.head = list2.mergeSort(list2.head);
 
// Get the difference list
const result = LinkedList.getDifference(list1.head, list2.head);
 
if (result.head !== null) {
  result.display();
} else {
  console.log("Lists are equal");
}

                    

Output
2 6 8







Time complexity: O(M Log M + N Log N).
 



Last Updated : 04 Dec, 2023
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