Java Program To Perform Union of Two Linked Lists Using Priority Queue

Given two linked lists, your task is to complete the function make union(), which returns the union of two linked lists. This union should include all the distinct elements only. The new list formed should be in non-decreasing order.

Input: L1 = 9->6->4->2->3->8
       L2 = 1->2->8->6->2
Output: 1 2 3 4 6 8 9

Approach:

There are two types of heap as we all know that is min heap and max heap 

• Min heap – Stores all the elements in ascending order
• Max heap – Stores all the elements in descending order

Let us first visualize using min heap in order to interpret program execution how union of two linked list is carried on, so de we are having two operations as listed to visualize:

1. Insertion
2. Removal

Insertion: After inserting all the distinct elements of two linked lists,

Removal: Removing the root until minheap is empty

Removing the root with value 1:

Removing the root with value 2:

Removing the root with value 3:

Removing the root with value 4:

Removing the root with value 6:

Removing the root with value 8:

Removing the root with value 9:

Hence we can conclude that in the min-heap, the smallest element will be at the root of the heap, and in the max heap, the greatest element will be at the root of the heap. While implementing the remove() function on heap, the root element will be removed. Since the output should be in increasing order, Min heap can be used. Priority Queue is used to implement min heap in java.  

Example

Java

// JAva Program toIllustrate Union of Two Linked Lists
// Using Priority Queue

// Importing basic required classes
import java.io.*;
import java.util.*;

// Class 1
// Helper class
// Node creation
class Node {

    // Data and addressing variable of node
    int data;
    Node next;

    // Constructor to initialize node
    Node(int a)
    {
        data = a;
        next = null;
    }
}

// Class 2
// Main class
public class GfG {
    // Reading input via Scanner class
    static Scanner sc = new Scanner(System.in);

    // Method 1
    // To create the input list1
    public static Node inputList1()
    {
        // Declaring node variables that is
        // Head and tail
        Node head, tail;

        // Custom input node elements

        head = tail = new Node(9);

        tail.next = new Node(6);
        // Fetching for next node
        // using next() method
        tail = tail.next;

        // Similarly for node 3
        tail.next = new Node(4);
        tail = tail.next;

        // Similarly for node 4
        tail.next = new Node(2);
        tail = tail.next;

        //  Similarly for node 5
        tail.next = new Node(3);
        tail = tail.next;

        // Similarly for node 6
        tail.next = new Node(8);
        tail = tail.next;

        // Returning the head
        return head;
    }

    // Method 2
    // To create the input List2
    // Similar to method 1 but for List2
    public static Node inputList2()
    {
        Node head, tail;

        head = tail = new Node(1);

        tail.next = new Node(2);
        tail = tail.next;

        tail.next = new Node(8);
        tail = tail.next;

        tail.next = new Node(6);
        tail = tail.next;

        tail.next = new Node(2);
        tail = tail.next;

        return head;
    }

    // Method 3
    // To print the union list
    public static void printList(Node n)
    {
        // Till there is a node
        // condition holds true
        while (n != null) {
            // Print the node
            System.out.print(n.data + " ");
            // Moving onto next node
            n = n.next;
        }
    }

    // Method 4
    // main driver method
    public static void main(String args[])
    {
        // Taking input for List 1 and List 2
        Node head1 = inputList1();
        Node head2 = inputList2();

        // Calling
        Union obj = new Union();
        printList(obj.findUnion(head1, head2));
    }
}

// Class 3
// To make the union of two linked list
class Union {
    public static Node findUnion(Node head1, Node head2)
    {
        // Creating a priority queue where
        // declaring elements of integer type
        PriorityQueue<Integer> minheap
            = new PriorityQueue<Integer>();
        // Setting heads
        Node l1 = head1, l2 = head2;

        // For List 1
        // Inserting elements from linked list1 into
        // priority queue
        while (l1 != null) {
            if (!minheap.contains(l1.data)) {
                minheap.add(l1.data);
            }
            // Move to next element
            l1 = l1.next;
        }

        // For List 2
        // Inserting elements from linked list2 into
        // priority queue
        while (l2 != null) {
            if (!minheap.contains(l2.data)) {
                minheap.add(l2.data);
            }

            // Move to next element
            l2 = l2.next;
        }

        Node union = new Node(0), start = union;

        // Removing until heap is empty
        while (!minheap.isEmpty()) {
            Node temp = new Node(minheap.remove());

            // Using temp to store start
            start.next = temp;
            start = start.next;
        }

        // Returning node
        return union.next;
    }
}

Output

1 2 3 4 6 8 9 

Time complexity: O(nlogn), Space complexity: O(n)