Mark Miyashita

public boolean lookup(Object element) {

}

Here’s the solution I came up with:

/*
 * This is the lookup method of the SinglyLinkedList class.
 */
public boolean lookup(Object element) {
  if (head == null) {
    return false;
  } else {
    return head.lookup(element);
  }
}

/*
 * This is the lookup method of the SinglyLinkedListNode class.
 */
public boolean lookup(Object element) {
  if (this.element == element) {
    return true;
  } else if (next == null) {
    return false;
  } else {
    return next.lookup(element);
  }
}

This implementation is another recursive function. First, we make sure that there’s a head to the list. If there isn’t, then the list definitely does not contain the element that we’re looking up. Then, we start looking at the head.

If the element is equal to the one that we’re looking for, then we can return true because we have found what we’re looking for. On the other hand, if the next element is null and we have not found what we’re looking for, then we should return false because the element was not found. Lastly, if we’re in the middle of the list and we have not yet found our element, we can continue to search the rest by calling lookup on the next item.

Here are some tests that you can add to the SinglyLinkedList class to verify that the lookup method is working. Of course, you should write more tests that what I have here, but it’s a start:

public static void main(String[] args) {
  Object[] elements = {3, 2, 1};
  SinglyLinkedList t = new SinglyLinkedList(elements);
  System.out.println(t.lookup(3));
  System.out.println(t.lookup(4));
}

Should print out:

true
false