Question

Im trying to write the methods to “union” which can be described: if A, B, C are sets, has the form C = A.union(B). Union returns a Set that contains all the elements in set A and B, but only list duplicates once.

My idea for this method is to traverse set A and add all of its elements to the union set, then traverse set B, if an element of set B is already present in the union set, then do not insert it to result, otherwise insert all to the union set.

This is complicated for a beginner like me since I want to include all 3 lists into the method (which I get a bunch of errors). I already wrote some methods in my SLinkedList class to check and add element but the parameters take an object from a Node

/** Singly linked list .*/
public class SLinkedList {

  protected Node head;   // head node of the list
  protected int size;    // number of nodes in the list

  /** Default constructor that creates an empty list */
  public SLinkedList() {
    head = new Node(null, null); // create a dummy head
    size = 0; 

 // add last
  public void addLast(Object data) {
      Node cur = head;
      // find last node
      while (cur.getNext() != null) {
          cur = cur.getNext();
      }
      // cur refers to the last node
      cur.setNext(new Node(data, null));
      size++;
  } 

 // contain method to check existing elements
  public boolean contain (Object target) {
      boolean status = false;
      Node cursor;
      for (cursor = head; cursor != null; cursor = cursor.getNext()) {
          if (target.equals(cursor.getElement())) {
              status = true;
          }       
      }   
      return status;
  }

  public SLinkedList union (SLinkedList secondSet) {
      SLinkedList unionSet = new SLinkedList();
      secondSet = new SLinkedList();
      Node cursor;
      for(cursor = head.getNext(); cursor != null; cursor = cursor.getNext()) {
          unionSet.addLast(cursor.getElement());
          // traverse secondSet, if an element is existed in either set A or union
              // set, skip, else add to union set
          }
      }
    return unionSet;
  }


  }

Node class

/** Node of a singly linked list of strings. */
public class Node {

  private Object element;   // we assume elements are character strings
  private Node next;

  /** Creates a node with the given element and next node. */
  public Node(Object o, Node n) {
    element = o;
    next = n;
  }

  /** Returns the element of this node. */
  public Object getElement() { return element; }

  /** Returns the next node of this node. */
  public Node getNext() { return next; }

  // Modifier methods:
  /** Sets the element of this node. */
  public void setElement(Object newElem) { element = newElem; }

  /** Sets the next node of this node. */
  public void setNext(Node newNext) { next = newNext; }

}

*update*
The question is if there is a second list involved public SLinkedList union (SLinkedList secondSet) , what syntax should i use to traverse set B and check if an element of set B is already present in result then do not insert it to result, otherwise insert. Do I need to creat a node for set B and traverse it?, there may be a compare method to compare the 2 sets outside of the union method?

Please help. Thanks all.

Answer 1

SLinkedList unionSet = null; // need a new SLinkedList() here
Node cursor;
for(cursor = head.getNext(); cursor != null; cursor = cursor.getNext()) {
    unionSet.addLast(cursor.getElement()); // NPE because unionSet is null
}