Stacks. Stacks. Main stack operations. The ADT Stack stores arbitrary objects. Insertions and deletions follow the last-in first-out (LIFO) principle.

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1 Stacks 1 Stacks The ADT Stack stores arbitrary objects. Insertions and deletions follow the last-in first-out (LIFO) principle. 2 Main stack operations Insertion and removal are defined by: push(e): inserts an element e. pop(): removes and returns the last inserted element. 3 1

2 Auxiliary stack operations In addition, there are: top(): returns the last inserted element without removing it. size(): returns the number of elements stored. isempty(): indicates whether no elements are stored. 4 Interface Stack In Java, the ADT stack is defined by: public interface Stack<E> { public int size(); public boolean isempty(); public void push(e e); public E top(); public E pop(); 5 Using Stack Example. With a stack, it is easy to reverse an array A of strings: Stack<String> S = new ArrayStack<String>(); for (int i = 0; i < A.length; i++) S.push(A[i]); for (int i = 0; i < A.length; i++) A[i] = S.pop(); 6 2

3 Using Stack Example. With a stack, it is easy to match parentheses of an expression such as {[(a + b)*c] + (d c)*e Declare a stack S. Scan a given expression from left to right. If (, [, { appears, push into S (, [, {, respectively. If ), ], appears, pop the top element from S and match. 7 Using Stack Example. With a stack, it is easy to match parentheses of an expression such as {[(a + b)*c] + (d c)*e Stack<Character> S = new ArrayStack<Character>(); for (int i = 0; i < exp.length(); i++) { if (exp.charat(i) == ( ) S.push( ( ); if (exp.charat(i) == ) ) if ((S.isEmpty()) (S.pop()!= ( )) return false; return (S.isEmpty()); 8 Array-based stacks A very simple way of implementing the ADT Stack. We add elements from left to right. A variable keeps track of the index of the top element. S t 9 3

4 Class ArrayStack public class ArrayStack<E> implements Stack<E> { public static final int CAPACITY = 1000; private E[] data; private int t = -1; public ArrayStack() { this(capacity); public ArrayStack(int capacity) { data = (E[]) new Object[capacity]; 10 Class ArrayStack public class ArrayStack<E> implements Stack<E> { public int size() { return (t + 1); public boolean isempty() { return (t == -1); public void push(e e) throws IllegalStateException { if (size() == data.length) throw new IllegalStateExcepion( Full ); t++; data[t] = e; 11 Class ArrayStack public class ArrayStack<E> implements Stack<E> { public E top() { if (isempty()) return null; return data[t]; public E pop() { if (isempty()) return null; E e = data[t]; t--; return e; 12 4

5 Class LinkedStack public class LinkedStack<E> implements Stack<E> { private SinglyLinkedList<E> list = new SinglyLinkedList(); public LinkedStack() { public int size() { return list.size(); public boolean isempty() { return list.isempty(); public void push(e e) { list.addfirst(e); public E top() { return list.first(); public E pop() { return list.removefirst(); 13 Queues 14 Queues The ADT Queue stores arbitrary objects. Insertion and removal follow the first-in first-out (FIFO) principle. Insertion occurs at the rear of the queue, and removal occurs at the front of the queue. 15 5

6 Applications of queues Waiting lists of all kinds. Managing access to shared resources (e.g., printers). Multiprogramming: allocation of a single system to more than one concurrent application, job or user. A component of another data structure. 16 Main queue operations Insertion and removal are defined by: enqueue(e): inserts an element e at the end of the queue. dequeue(): removes and returns the element at the front of the queue. 17 Auxiliary queue operations In addition, there are: first(): returns the element at the front of the queue without removing it. size(): returns the number of elements stored. isempty(): indicates whether no elements are stored. 18 6

7 Interface Queue In Java, the ADT queue is defined by: public interface Queue<E> { public int size(); public boolean isempty(); public void enqueue(e e); public E first(); public E dequeue(); 19 Array-based queues We use an array in a circular fashion. f refers to the index of the front element. r refers to the index immediately past the rear element for insertion. Q Q A normal configuration f r A wrapped-around configuration r f 20 Class ArrayQueue public class ArrayQueue<E> implements Queue<E> { public static final int CAPACITY = 1000; private E[] data; private int f = 0; private int sz = 0; public ArrayQueue() { this(capacity); public ArrayQueue(int capacity) { data = (E[]) new Object[capacity]; 21 7

8 Class ArrayQueue public class ArrayQueue<E> implements Queue<E> { public int size() { return sz; public boolean isempty() { return (sz == 0); public void enqueue(e e) throws IllegalStateException { if (size() == data.length) throw new IllegalStateExcepion( Full ); int r = (f + sz) % data.length; data[r] = e; sz++; 22 Class ArrayQueue public class ArrayQueue<E> implements Queue<E> { public E first() { if (isempty()) return null; return data[f]; public E dequeue() { if (isempty()) return null; E e = data[f]; f = (f + 1) % data.length; sz--; return e; 23 Class LinkedQueue public class LinkedQueue<E> implements Queue<E> { private SinglyLinkedList<E> list = new SinglyLinkedList(); public LinkedQueue() { public int size() { return list.size(); public boolean isempty() { return list.isempty(); public void enqueue(e e) { list.addlast(e); public E first() { return list.first(); public E dequeue() { return list.removefirst(); 24 8

Stacks and Queues. EECS2030: Advanced Object Oriented Programming Fall 2017 CHEN-WEI WANG

Stacks and Queues. EECS2030: Advanced Object Oriented Programming Fall 2017 CHEN-WEI WANG Stacks and Queues EECS2030: Advanced Object Oriented Programming Fall 2017 CHEN-WEI WANG What is a Stack? A stack is a collection of objects. Objects in a stack are inserted and removed according to the