1. The List ADT
Definition A list is a collection of objects, called nodes, connected in a chain
by links. There may or may not be an ordering relationship between data in the
nodes. Depending on this, we distinguish between ordered and unordered
lists. For now, we consider only unordered lists.
Operations (methods) on lists:
insertFirst (node) Inserts a new node at the beginning of the list
deleteFirst (node) Deletes a node from the beginning of the list
size () Returns the number of nodes in the list
empty () Returns true if the list is empty
search (key) Searches for a node with the specified key
traverse () Processes each node on the list (for example,
prints it out)

2. A linked list interface and the Node class
public interface LList {
public void insertFirst (int item);
public Node deleteFirst ();
public int size();
public boolean empty ();
public boolean search (int item);
public void traverse (); }
class Node {
private int data;
private Node next;
public Node () {
this(0, null); }
public Node (int d) {
data = d; }
public Node (int d, Node n) {
data = d;
next = n; }
public void setData (int newData) {
data = newData; }
public void setNext (Node newNext) {
next = newNext; }
public int getData () {
return data; }
public Node getNext () {
return next; }
public void displayNode () {
System.out.print (data); } }

3. Implementation of an unordered linked list ADT
public Node deleteFirst () {
Node temp = first;
first = first.getNext();
return temp; }
public boolean search (int key) {
boolean result = false;
Node current = first;
while (current != null) {
if (current.getData () == key) {
result = true;
return result; }
else
current = current.getNext(); }
return result; }
public void traverse () {
System.out.print ("Current list: ");
current.displayNode ();
System.out.print (" ");
current = current.getNext(); }
System.out.println (); }
class LinkListADT implements LList {
private Node first;
public LinkListADT () {
first = null; }
public boolean empty () {
return (first == null); }
public int size () {
int count = 0;
Node current = first;
while (current != null) {
count++;
current = current.getNext(); }
return count;
public void insertFirst (int newData) {
Node newFirst = new Node (newData);
newFirst.setNext(first);
first = newFirst;

4. Efficiency of linked list operations
empty, insertFirst and deleteFirst methods are O(1), while size, search and
traverse methods are O(N). The size method can be rewritten in a more
efficient, O(1), form, but the (linear) search and traverse methods do require
a loop to process each of the nodes on the list.
If we want to insert a node in a place different than the beginning of the list,
we must first find that place by means of the search method, and then insert
(or delete) the node. Although the actual insert / delete process involves
simply an exchange of two pointers, the overall efficiency of these method
will be O(N) because of the search involved.
Ordered linked lists rely on an insertN method to add the new node in the
appropriate place according to the prescribed ordering relationship. Therefore,
the insertion in an ordered linked list will also be O(N) operation (if linear
search is used).

5. The Queue ADT -- a linked list implementation
public interface LLQueue {
public void enqueue (int item);
public int dequeue();
public int size();
public boolean empty();
public int front(); }
class LLQueueADT implements LLQueue {
private int size;
private Node front;
private Node rear;
public LLQueueADT () {
size = 0;
front = null;
rear = null;
public boolean empty () {
return (size == 0);
public int size () {
return size;
public void enqueue (int number) {
Node newNode = new Node ();
newNode.setData(number);
newNode.setNext(null);
if (this.empty())
front = newNode;
else
rear.setNext(newNode);
rear = newNode;
size++; }
public int dequeue () {
int i;
i = front.getData();
front = front.getNext();
size--;
rear = null;
return i;
public int front () {
return front.getData();

6. The Stack ADT -- a linked list implementation
public interface LLStack {
public void push (int item);
public int pop();
public int size();
public boolean empty();
public int ontop(); }
class LLStackADT implements LLStack {
private Node top;
private int size;
public LLStackADT () {
top = null;
size = 0;
public boolean empty () {
return (top == null);
public int size () {
return size;
public void push (int number) {
Node newNode = new Node ();
newNode.setData(number);
newNode.setNext(top);
top = newNode;
size++; }
public int pop () {
int i;
i = top.getData();
top = top.getNext();
size--;
return i;
public int ontop () {
int i = pop();
push(i);

7. Double-ended Queues
The double-ended queue (or dequeu) supports insertions and deletions at the
front and at the rear of the queue. Therefore, in addition to enqueue, dequeue
(which we call now insertLast and deleteFirst, respectively), and first
methods, we must also provide insertFirst, deleteFirst and last methods.
The Dequeue ADT can be used for implementing both stacks and queues.
Dequeue methods Stack methods Queue methods
size() size() size()
empty() empty() empty()
last() top()
first() front()
insertFirst()
insertLast() push(item) enqueue(item)
deleteLast() pop()
deleteFirst() dequeue()

8. An implementation of a doubly linked list
Nodes in a doubly linked list have pointers to both, the next node and the previous
node. For simplicity, we can add two dummy nodes to the list: the header node,
which comes before the first node, and the trailer node, which comes after the
last node.
The doubly linked list ADT has the following interface:
public interface DLList {
public void insertFirst (int item);
public void insertLast (int item);
public DLNode deleteFirst ();
public DLNode deleteLast ();
public int size();
public int last ();
public int first ();
public boolean empty ();
public void traverse (); }

9. Implementation of a doubly linked list (cont.)
class DLNode {
private int data;
private DLNode next, prev;
public DLNode () {
this(0, null, null); }
public DLNode (int d) {
data = d;
next = null;
prev = null;
public DLNode (int newData, DLNode newNext, DLNode newPrev) {
data = newData;
next = newNext;
prev = newPrev;
public void setData (int newData) {
public void setNext (DLNode newNext) {
next = newNext; }
public void setPrev (DLNode newPrev) {
prev = newPrev;
public int getData () {
return data;
public DLNode getNext () {
return next;
public DLNode getPrev () {
return prev;
public void displayDLNode () {
System.out.print (data);

10. Implementation of a doubly linked list (cont.)
class DLListADT implements DLList {
private DLNode header;
private DLNode trailer;
private int size;
public DLListADT () {
header = new DLNode();
trailer = new DLNode();
header.setNext(trailer);
header.setPrev(null);
header.setData(0);
trailer.setPrev(header);
trailer.setNext(null);
trailer.setData(0);
size = 0; }
public boolean empty () {
return (size == 0); }
public int size () {
return size; }
public void insertFirst (int newData) {
DLNode oldFirst = header.getNext();
DLNode newFirst = new DLNode (newData, oldFirst, header);
oldFirst.setPrev(newFirst);
header.setNext(newFirst);
size++; }
public void insertLast (int newData) {
DLNode oldLast = trailer.getPrev();
DLNode newLast = new DLNode (newData, trailer, oldLast);
oldLast.setNext(newLast);
trailer.setPrev(newLast);
public DLNode deleteFirst () {
DLNode newFirst = oldFirst.getNext();
newFirst.setPrev(header);
size--;
return oldFirst; }

11. Implementation of a doubly linked list (cont.)
public DLNode deleteLast () {
DLNode oldLast = trailer.getPrev();
DLNode newLast = oldLast.getPrev();
trailer.setPrev(newLast);
newLast.setNext(trailer);
size--;
return oldLast; }
public boolean search (int key) {
boolean result = false;
DLNode current = header.getNext();
while (current != trailer) {
if (current.getData () == key) {
result = true;
return result;
else
current = current.getNext();
public int last () {
return (trailer.getPrev().getData()); }
public int first () {
return (header.getNext().getData());
public void traverse () {
System.out.print ("Current list: ");
DLNode current = header.getNext();
while (current != trailer) {
current.displayDLNode ();
System.out.print (" ");
current = current.getNext();
System.out.println ();