1. Chapter 7 Stacks II CS2006 - Data Structures I COSC 2006
April 22, 2017
CS Data Structures I
Chapter 7 Stacks II
Chapter 6: Stacks

2. Topics
Implementation of ADT Stack
Using Linked lists
Using ADT List

3. Linked List Stack Implementation
COSC 2006
April 22, 2017
Linked List Stack Implementation
Stacks can grow and shrink dynamically
No particular linked list variations are required:
The head pointer can also serve as the top of the stack.
Pushing becomes inserting the first node in a linked list.
Popping becomes removing the first node.
Chapter 6: Stacks

4. Linked List Stack Implementation
COSC 2006
April 22, 2017
Linked List Stack Implementation
The head of List is the top of the stack
head 1 3 7 9 1
top 3 7 9
Chapter 6: Stacks

5. LL Stack Implementation
COSC 2006
April 22, 2017
LL Stack Implementation
// Assumes that the classes StackInterface and Node are available
public class StackRefereenceBased implements StackInterface {
private Node top;
// class Node is the same used in the LinkedList class except it stores
// Objects rather than Comparables
public StackRefereenceBased (){
top = null;
} // end default constructor
public boolean isEmpty() {
return (top == null);
} // end isEmpty
Chapter 6: Stacks

6. LL Stack Implementation
COSC 2006
April 22, 2017
LL Stack Implementation
// Assumes that the classes StackInterface and Node are available
public class StackRefereenceBased implements StackInterface {
private Node top;
// class Node is the same used in the LinkedList class except it stores
// Objects rather than Comparables
public StackRefereenceBased (){
top = null;
} // end default constructor
public boolean isEmpty() {
return (top == null);
} // end isEmpty
Chapter 6: Stacks

7. LL Stack Implementation (2)
COSC 2006
April 22, 2017
LL Stack Implementation (2)
public void push(Object newItem) {
top = new Node(newItem, top);
} // end push
public Object pop() throws StackException {
if (!isEmpty()) {
Object temp = top.getItem();
top = top.getNext();
return temp; }
else {
throw new StackException("StackException on " + "pop: stack empty");
} // end if
} // end pop
Chapter 6: Stacks

8. LL Stack Implementation (2)
COSC 2006
April 22, 2017
LL Stack Implementation (2)
public void push(Object newItem) {
top = new Node(newItem, top);
} // end push
public Object pop() throws StackException {
if (!isEmpty()) {
Object temp = top.getItem();
top = top.getNext();
return temp; }
else {
throw new StackException("StackException on " + "pop: stack empty");
} // end if
} // end pop
Chapter 6: Stacks

9. LL Stack Implementation (3)
COSC 2006
April 22, 2017
LL Stack Implementation (3)
public void popAll() {
top=null;
} // end popAll
public Object peek() throws StackException {
if (!isEmpty()) {
return top.getItem(); }
else {
throw new StackException("StackException on " +
"peek: stack empty");
} // end else
} // end peek
} // end StackReferencedBased
Chapter 6: Stacks

10. LL Stack Implementation (3)
COSC 2006
April 22, 2017
LL Stack Implementation (3)
public void popAll() {
top=null;
} // end popAll
public Object peek() throws StackException {
if (!isEmpty()) {
return top.getItem(); }
else {
throw new StackException("StackException on " +
"peek: stack empty");
} // end else
} // end peek
} // end StackReferencedBased
Chapter 6: Stacks

11. ADT Stack ADT List Implementation
All stack operations can be implemented using the ADT List's operations
The implementation is simpler to write than the array-based and makes used of previously-written code (code reuse)
Position 1 of the list represents the top position of the stack

12. ADT Stack ADT List Implementation
top
List position 10 80 60 . 5 1 2 3
List site()

13. ADT List Stack Implementation
COSC 2006
April 22, 2017
ADT List Stack Implementation
// Assumes that the classes ListInterface and Node are available
public class StackListBased implements StackInterface {
private ListInterface list;
public StackListBased() {
list = new ListReferenceBased();
} // end default constructor
public boolean isEmpty() {
return list.isEmpty ();
} // end isEmpty
Chapter 6: Stacks

14. ADT List Stack Implementation
COSC 2006
April 22, 2017
ADT List Stack Implementation
// Assumes that the classes ListInterface and Node are available
public class StackListBased implements StackInterface {
private ListInterface list;
public StackListBased() {
list = new ListReferenceBased();
} // end default constructor
public boolean isEmpty() {
return list.isEmpty ();
} // end isEmpty
Chapter 6: Stacks

15. List Stack Implementation (2)
COSC 2006
April 22, 2017
List Stack Implementation (2)
public void push(Object newItem) {
list.add (1, newItem);
} // end push
public Object pop() throws StackException {
if (!list.isEmpty()) {
Object temp = list.get(1);
list.remove(1);
return temp; }
else {
throw new StackException("StackException on " + "pop: stack empty");
} // end if
} // end pop
Chapter 6: Stacks

16. List Stack Implementation (2)
COSC 2006
April 22, 2017
List Stack Implementation (2)
public void push(Object newItem) {
list.add (1, newItem);
} // end push
public Object pop() throws StackException {
if (!list.isEmpty()) {
Object temp = list.get(1);
list.remove(1);
return temp; }
else {
throw new StackException("StackException on " + "pop: stack empty");
} // end if
} // end pop
Chapter 6: Stacks

17. List Stack Implementation (3)
COSC 2006
April 22, 2017
List Stack Implementation (3)
public void popAll() {
list.removeAll();
} // end popAll
public Object peek() throws StackException {
if (!list.isEmpty()) {
return list.get(1); }
else {
throw new StackException("StackException on " +
"peek: stack empty");
} // end else
} // end peek
} // end StackListBased
Chapter 6: Stacks

18. ADT Stack Implementation Comparison
Array-Based
  No overhead of reference manipulation
  Prevents adding elements if the array is full
LL-Based
More complicated than ADT List
Most flexible, No size restrictions
ADT List-Based
Simpler to write
Not as efficient as using a linked list directly

19. Review
The ______ method of the ADT stack retrieves and then removes the top of the stack.
createStack
push
pop
peek

20. Review
The ______ method of the ADT stack retrieves the top of the stack, but does not change the stack.
createStack
push
pop
peek

21. Review
Which of the following methods of the ADT stack accepts a parameter?
push
pop
createStack
peek

22. Review
If a stack is used by an algorithm to check for balanced braces, which of the following is true once the end of the string is reached?
the stack is empty
the stack has one “{”
the stack has one “}”
the stack has one “{” and one “}”

23. Review
Which of the following operations of the ADT stack does not throw a StackException?
push
pop
popAll
peek

24. Review
The pop operation throws a StackException when it tries to ______.
add an item to an empty stack
add an item to an array-based implementation of a stack that is already full
delete an item from an array-based implementation of a stack that is already full
delete an item from an empty stack

25. Review
The push operation throws a StackException when it tries to ______.
add an item to an empty stack
add an item to an array-based implementation of a stack that is already full
delete an item from an array-based implementation of a stack that is already full
delete an item from an empty stack

26. Review
In the StackInterface class, the pop method returns an item that is an instance of ______.
Integer
Double
String
Object