Java Program For Union And Intersection Of Two Linked Lists

Last Updated : 12 Oct, 2023

Write a java program for a given two Linked Lists, create union and intersection lists that contain union and intersection of the elements present in the given lists. The order of elements in output lists doesn’t matter.
Example:

Input:
List1: 10->15->4->20
List2:  8->4->2->10
Output:
Intersection List: 4->10
Union List: 2->8->20->4->15->10

Method 1 (Simple):

The following are simple algorithms to get union and intersection lists respectively.
Intersection (list1, list2)
Initialize the result list as NULL. Traverse list1 and look for every element in list2, if the element is present in list2, then add the element to the result.
Union (list1, list2):
Initialize a new list ans and store first and second list data to set to remove duplicate data
and then store it into our new list ans and return its head.

Below is the implementation of above approach:

Java

// Java program to find union and
// intersection of two unsorted
// linked lists
class LinkedList {
    Node head; // head of list
 
    /* Linked list Node*/
    class Node {
        int data;
        Node next;
        Node(int d)
        {
            data = d;
            next = null;
        }
    }
 
    /* Function to get Union of 2 Linked Lists */
    void getUnion(Node head1, Node head2)
    {
        Node t1 = head1, t2 = head2;
 
        // insert all elements of list1 in the result
        while (t1 != null) {
            push(t1.data);
            t1 = t1.next;
        }
 
        // insert those elements of list2
        // that are not present
        while (t2 != null) {
            if (!isPresent(head, t2.data))
                push(t2.data);
            t2 = t2.next;
        }
    }
 
    void getIntersection(Node head1, Node head2)
    {
        Node result = null;
        Node t1 = head1;
 
        // Traverse list1 and search each
        // element of it in list2.
        // If the element is present in
        // list 2, then insert the
        // element to result
        while (t1 != null) {
            if (isPresent(head2, t1.data))
                push(t1.data);
            t1 = t1.next;
        }
    }
 
    /* Utility function to print list */
    void printList()
    {
        Node temp = head;
        while (temp != null) {
            System.out.print(temp.data + " ");
            temp = temp.next;
        }
        System.out.println();
    }
 
    /* Inserts a node at start of linked list */
    void push(int new_data)
    {
        /* 1 & 2: Allocate the Node &
                        Put in the data*/
        Node new_node = new Node(new_data);
 
        /* 3. Make next of new Node as head */
        new_node.next = head;
 
        /* 4. Move the head to point to new Node */
        head = new_node;
    }
 
    /* A utility function that returns true
    if data is present in linked list
    else return false */
    boolean isPresent(Node head, int data)
    {
        Node t = head;
        while (t != null) {
            if (t.data == data)
                return true;
            t = t.next;
        }
        return false;
    }
 
    /* Driver program to test above functions */
    public static void main(String args[])
    {
        LinkedList llist1 = new LinkedList();
        LinkedList llist2 = new LinkedList();
        LinkedList unin = new LinkedList();
        LinkedList intersecn = new LinkedList();
 
        /*create a linked lists 10->15->4->20 */
        llist1.push(20);
        llist1.push(4);
        llist1.push(15);
        llist1.push(10);
 
        /*create a linked lists 8->4->2->10 */
        llist2.push(10);
        llist2.push(2);
        llist2.push(4);
        llist2.push(8);
 
        intersecn.getIntersection(llist1.head, llist2.head);
        unin.getUnion(llist1.head, llist2.head);
 
        System.out.println("First List is");
        llist1.printList();
 
        System.out.println("Second List is");
        llist2.printList();
 
        System.out.println("Intersection List is");
        intersecn.printList();
 
        System.out.println("Union List is");
        unin.printList();
    }
} /* This code is contributed by Rajat Mishra */

Output:

First list is 
10 15 4 20 
Second list is 
8 4 2 10 
Intersection list is 
4 10 
Union list is 
2 8 20 4 15 10

Complexity Analysis:

Time Complexity: O(m*n).
Here ‘m’ and ‘n’ are number of elements present in the first and second lists respectively.
For union: For every element in list-2 we check if that element is already present in the resultant list made using list-1.
For intersection: For every element in list-1 we check if that element is also present in list-2.
Auxiliary Space: O(1).
No use of any data structure for storing values.

Method 2 (Use Merge Sort):

In this method, algorithms for Union and Intersection are very similar. First, we sort the given lists, then we traverse the sorted lists to get union and intersection.
The following are the steps to be followed to get union and intersection lists.

Sort the first Linked List using merge sort. This step takes O(mLogm) time. Refer this post for details of this step.

Sort the second Linked List using merge sort. This step takes O(nLogn) time. Refer this post for details of this step.

Linearly scan both sorted lists to get the union and intersection. This step takes O(m + n) time. This step can be implemented using the same algorithm as sorted arrays algorithm discussed here.

Below is the implementation of above approach:

Java

class Node {
    int data;
    Node next;
 
    Node(int data)
    {
        this.data = data;
        this.next = null;
    }
}
 
public class LinkedListUnion {
 
    static class Node {
        int data;
        Node next;
        Node(int d)
        {
            data = d;
            next = null;
        }
    }
 
    // function to print linked list
    public static void printLinkedList(Node head)
    {
        Node temp = head;
        while (temp != null) {
            System.out.print(temp.data + "-->");
            temp = temp.next;
        }
        System.out.print("None");
    }
 
    // function to get union of two linked lists
    static Node getUnion(Node ll1, Node ll2)
    {
        Node tail = null;
        Node head = null;
        while (ll1 != null || ll2 != null) {
            if (ll1 == null) {
                if (tail == null) {
                    tail = new Node(ll2.data);
                    head = tail;
                }
                else {
                    tail.next = new Node(ll2.data);
                    tail = tail.next;
                }
                ll2 = ll2.next;
            }
            else if (ll2 == null) {
                if (tail == null) {
                    tail = new Node(ll1.data);
                    head = tail;
                }
                else {
                    tail.next = new Node(ll1.data);
                    tail = tail.next;
                }
                ll1 = ll1.next;
            }
            else {
                if (ll1.data < ll2.data) {
                    if (tail == null) {
                        tail = new Node(ll1.data);
                        head = tail;
                    }
                    else {
                        tail.next = new Node(ll1.data);
                        tail = tail.next;
                    }
                    ll1 = ll1.next;
                }
                else if (ll1.data > ll2.data) {
                    if (tail == null) {
                        tail = new Node(ll2.data);
                        head = tail;
                    }
                    else {
                        tail.next = new Node(ll2.data);
                        tail = tail.next;
                    }
                    ll2 = ll2.next;
                }
                else {
                    if (tail == null) {
                        tail = new Node(ll1.data);
                        head = tail;
                    }
                    else {
                        tail.next = new Node(ll1.data);
                        tail = tail.next;
                    }
                    ll1 = ll1.next;
                    ll2 = ll2.next;
                }
            }
        }
        return head;
    }
 
    // main function to test the code
    public static void main(String[] args)
    {
        // create first linked list
        Node head1 = new Node(10);
        head1.next = new Node(20);
        head1.next.next = new Node(30);
        head1.next.next.next = new Node(40);
        head1.next.next.next.next = new Node(50);
        head1.next.next.next.next.next = new Node(60);
        head1.next.next.next.next.next.next = new Node(70);
 
        // create second linked list
        Node head2 = new Node(10);
        head2.next = new Node(30);
        head2.next.next = new Node(50);
        head2.next.next.next = new Node(80);
        head2.next.next.next.next = new Node(90);
 
        Node head = getUnion(head1, head2);
        printLinkedList(head);
        System.out.println();
    }
}
// This code is contributed by Vikram_Shirsat

Output

10-->20-->30-->40-->50-->60-->70-->80-->90-->None

Method 3 (Use Hashing):

1. Union (list1, list2):
Initialize the result list as NULL and create an empty hash table. Traverse both lists one by one, for each element being visited, look at the element in the hash table. If the element is not present, then insert the element into the result list. If the element is present, then ignore it.
2. Intersection (list1, list2)
Initialize the result list as NULL and create an empty hash table. Traverse list1. For each element being visited in list1, insert the element in the hash table. Traverse list2, for each element being visited in list2, look the element in the hash table. If the element is present, then insert the element to the result list. If the element is not present, then ignore it.
Both of the above methods assume that there are no duplicates.

Below is the implementation of above approach:

Java

// Java code for Union and Intersection of two
// Linked Lists
import java.util.HashMap;
import java.util.HashSet;
 
class LinkedList {
    Node head; // head of list
 
    /* Linked list Node*/
    class Node {
        int data;
        Node next;
        Node(int d)
        {
            data = d;
            next = null;
        }
    }
 
    /* Utility function to print list */
    void printList()
    {
        Node temp = head;
        while (temp != null) {
            System.out.print(temp.data + " ");
            temp = temp.next;
        }
        System.out.println();
    }
 
    /* Inserts a node at start of linked list */
    void push(int new_data)
    {
        /* 1 & 2: Allocate the Node &
        Put in the data*/
        Node new_node = new Node(new_data);
 
        /* 3. Make next of new Node as head */
        new_node.next = head;
 
        /* 4. Move the head to point to new Node */
        head = new_node;
    }
 
    public void append(int new_data)
    {
        if (this.head == null) {
            Node n = new Node(new_data);
            this.head = n;
            return;
        }
        Node n1 = this.head;
        Node n2 = new Node(new_data);
        while (n1.next != null) {
            n1 = n1.next;
        }
 
        n1.next = n2;
        n2.next = null;
    }
 
    /* A utility function that returns true if data is
    present in linked list else return false */
    boolean isPresent(Node head, int data)
    {
        Node t = head;
        while (t != null) {
            if (t.data == data)
                return true;
            t = t.next;
        }
        return false;
    }
 
    LinkedList getIntersection(Node head1, Node head2)
    {
        HashSet<Integer> hset = new HashSet<>();
        Node n1 = head1;
        Node n2 = head2;
        LinkedList result = new LinkedList();
 
        // loop stores all the elements of list1 in hset
        while (n1 != null) {
            if (hset.contains(n1.data)) {
                hset.add(n1.data);
            }
            else {
                hset.add(n1.data);
            }
            n1 = n1.next;
        }
 
        // For every element of list2 present in hset
        // loop inserts the element into the result
        while (n2 != null) {
            if (hset.contains(n2.data)) {
                result.push(n2.data);
            }
            n2 = n2.next;
        }
        return result;
    }
 
    LinkedList getUnion(Node head1, Node head2)
    {
        // HashMap that will store the
        // elements of the lists with their counts
        HashMap<Integer, Integer> hmap = new HashMap<>();
        Node n1 = head1;
        Node n2 = head2;
        LinkedList result = new LinkedList();
 
        // loop inserts the elements and the count of
        // that element of list1 into the hmap
        while (n1 != null) {
            if (hmap.containsKey(n1.data)) {
                int val = hmap.get(n1.data);
                hmap.put(n1.data, val + 1);
            }
            else {
                hmap.put(n1.data, 1);
            }
            n1 = n1.next;
        }
 
        // loop further adds the elements of list2 with
        // their counts into the hmap
        while (n2 != null) {
            if (hmap.containsKey(n2.data)) {
                int val = hmap.get(n2.data);
                hmap.put(n2.data, val + 1);
            }
            else {
                hmap.put(n2.data, 1);
            }
            n2 = n2.next;
        }
 
        // Eventually add all the elements
        // into the result that are present in the hmap
        for (int a : hmap.keySet()) {
            result.append(a);
        }
        return result;
    }
 
    /* Driver program to test above functions */
    public static void main(String args[])
    {
        LinkedList llist1 = new LinkedList();
        LinkedList llist2 = new LinkedList();
        LinkedList union = new LinkedList();
        LinkedList intersection = new LinkedList();
 
        /*create a linked list 10->15->4->20 */
        llist1.push(20);
        llist1.push(4);
        llist1.push(15);
        llist1.push(10);
 
        /*create a linked list 8->4->2->10 */
        llist2.push(10);
        llist2.push(2);
        llist2.push(4);
        llist2.push(8);
 
        intersection = intersection.getIntersection(
            llist1.head, llist2.head);
        union = union.getUnion(llist1.head, llist2.head);
 
        System.out.println("First List is");
        llist1.printList();
 
        System.out.println("Second List is");
        llist2.printList();
 
        System.out.println("Intersection List is");
        intersection.printList();
 
        System.out.println("Union List is");
        union.printList();
    }
}
// This code is contributed by Kamal Rawal

Output:

First List is
10 15 4 20 
Second List is
8 4 2 10 
Intersection List is
10 4 
Union List is
2 4 20 8 10 15

Complexity Analysis:

Time Complexity: O(m+n).
Here ‘m’ and ‘n’ are number of elements present in the first and second lists respectively.
For union: Traverse both the lists, store the elements in Hash-map and update the respective count.
For intersection: First traverse list-1, store its elements in Hash-map and then for every element in list-2 check if it is already present in the map. This takes O(1) time.
Auxiliary Space:O(m+n).
Use of Hash-map data structure for storing values.

Please refer complete article on Union and Intersection of two Linked Lists for more details!