Solution: Union and Intersection of Lists Using Hashing

class UnionIntersectionChallenge {
    public static < T > SinglyLinkedList < T > unionWithHashing(SinglyLinkedList < T > list1, SinglyLinkedList < T > list2) {
        SinglyLinkedList < T > result = new SinglyLinkedList < T > ();
        HashSet < T > visited = new HashSet < T > ();
        SinglyLinkedList < T > .Node current = list1.getHeadNode(); //start from list1's head
        //Keep iterating list1
        while (current != null) {
            //add elements of list1 into the result if they havent been visited before
            if (!visited.contains(current.data)) {
                result.insertAtHead(current.data);
                visited.add(current.data);
            }
            current = current.nextNode;
        }
        //now start from the head of list2
        current = list2.getHeadNode();
        //Keep iterating list2
        while (current != null) {
            //add elements of list2 into the result if they havent been visited before
            if (!visited.contains(current.data)) {
                result.insertAtHead(current.data);
                visited.add(current.data);
            }
            current = current.nextNode;
        }
        return result;
    }
    public static < T > SinglyLinkedList < T > intersectionWithHashing(SinglyLinkedList < T > list1, SinglyLinkedList < T > list2) {
        SinglyLinkedList < T > result = new SinglyLinkedList < T > ();
        HashSet < T > visited = new HashSet < T > ();
        //start from the head of list1
        SinglyLinkedList < T > .Node current = list1.getHeadNode();
        //Keep iterating list1
        while (current != null) {
            //Add elements to visited set if they have not been visited
            if (!visited.contains(current.data)) {
                visited.add(current.data);
            }
            current = current.nextNode;
        }
        //now take head of list2
        current = list2.getHeadNode();
        //iterate list2
        while (current != null) {
            //add the elements which have been visited before into the result
            if (visited.contains(current.data)) {
                result.insertAtHead(current.data);
                visited.remove(current.data);
            }
            current = current.nextNode;
        }
        return result;
    }
    public static void main(String args[]) {
        SinglyLinkedList < Integer > list1 = new SinglyLinkedList < Integer > ();
        for (int i = 7; i > 3; i--) {
            list1.insertAtHead(i);
        }
        System.out.print("1st ");
        list1.printList();
        SinglyLinkedList < Integer > list2 = new SinglyLinkedList < Integer > ();
        for (int i = 0; i < 5; i++) {
            list2.insertAtHead(i);
        }
        System.out.print("2nd ");
        list2.printList();
        System.out.print("After Intersection ");
        intersectionWithHashing(list1, list2).printList();
        System.out.print("After Union ");
        unionWithHashing(list1, list2).printList();
    }
}