1. COMP 103
Linked Stack
and
Linked Queue

2. RECAP-TODAY RECAP TODAY Linked data structures: Linked Node
LinkedList class
TODAY
Linked Stack
Linked Queue
Other linked collections

3. A Linked Stack Implement a Stack using a linked list.
Which end is the top of the stack?
pop()
push(“A”)
Why is this a bad idea?
data M J P C X
TOP
O(n) A H

4. A Linked Stack Make the top of the Stack be the head of the list. TOP
pop()
push(“A”)
data M
TOP J P C X A H

5. Implementing LinkedStack
Implementing a stack using LinkedNode as underlying data structure (instead of array)
public class LinkedStack <E> extends AbstractCollection<E> {
private LinkedNode<E> data = null;
public LinkedStack() {  } // constructor
public int size() { … }
public boolean empty() { … }
public E peek() { … } // Usually called top
public E pop() { … }
public void push(E item) { … }
public int search(Object o) { … }
// plus others inherited from AbstractCollection, such as toArray() and others... }
data

6. More LinkedStack methods
public boolean empty() {
return data == null; }
public E peek() {
if (data == null) throw new EmptyStackException(); return data.get();
public E pop() {
if (data == null) throw new EmptyStackException(); E ans = data.get(); data = data.next(); return ans;
public void push(E item) {
if (item == null) throw new IllegalArgumentException(); data = new LinkedNode(item, data);
data M H
data
does 3 jobs...

7. Iterator (for any list)
Iterating down a stack.
for (String str: myStack)
System.out.println(str)
myStack M X
data P J C
iterator
node H

8. Iterator
public Iterator <E> iterator() {     return new LinkedNodeIterator(data); }
private class LinkedNodeIterator <E> implements Iterator <E> {
private LinkedNode<E> node;     // node containing next item
public LinkedNodeIterator (LinkedNode <E> node) {
this.node = node;
public boolean hasNext () {
return (node != null);
public E next () {
if (node == null) throw new NoSuchElementException();
E ans = node.get();
node = node.next();
return ans;
public void remove () {
throw new UnsupportedOperationException();

9. A Linked Queue #1 Put the front of the queue at the end of the list
offer(“A”) O(1)
poll() O(n)
back M A J P C H X
Front

10. A Linked Queue #2 Put the front of the Queue at the start of the list.
poll() O(1)
offer(“A”) O(n) is too Slow!!
Front M J P C X H
Back A

11. A Better Linked Queue Keep pointers to both ends! poll() O(1)
offer(“A”) O(1) M P
Front
Back X J C H
Back A

12. Implementing LinkedQueue
public class LinkedQueue <E> implements AbstractQueue <E> { private LinkedNode<E> front = null; private LinkedNode<E> back = null; public LinkedQueue() {  }
public int size() { …
public boolean isEmpty() { …
public E peek() { …
public E poll() { …
public void offer(E item) { …
public Iterator <E> iterator() { … M
Front
Back A

13. LinkedQueue public boolean isEmpty() { return front == null; }
public int size () { if (front == null)   return 0; else   return front.size();
public E peek() { if (front == null) return null; else return front.get();
Front
Back
Front
Back M A

14. LinkedQueue three cases: 0 items, 1 item, >1 items
public E poll() {
if (front == null) return null;
E ans = front.get();
front = front.next();
if (front == null) back = null;
return ans; }
Front
Back
three cases:
0 items,
1 item,
>1 items
Front
Back M
Front
Back A A

15. LinkedQueue three cases: >1 item, 1 item, 0 items
public boolean offer(E item) { if (item == null) return false; if (front == null) {     back = new LinkedNode(item, null);     front = back; } else {     back.setNext(new LinkedNode(item, null));     back= back.next(); } return true; }
Front
Back M A
three cases:
>1 item,
1 item,
0 items
Front
Back
Front
Back A

16. Linked Stacks and Queues
Use a “header node”
Keep link to head node, and maybe last node of linked list
Important to choose the right end.
Easy to add or remove from head of a linked list
Easy to add to last node of linked list (if have pointer to tail too)
Hard to remove the last node of linked list (do you see why?)
Linked Stack and Queue:
all main operations are O(1)
Can combine Stack and Queue giving Dequeue.
addFirst, addLast, removeFirst (or pushLeft, pushRight, popLeft)
How can we make all operations fast?
See the java “LinkedList” class.

17. Other linked collections
Set
Can store sorted or unsorted
What is the cost of add, remove, contains, size, equals in each case?
Bag
Same as set, but add a count to each node.
Map
Same as set, but store value as well as key in each node
Or store pointed to a key-value pair
What is the cost of get, put, remove?