cmsc 132: object-oriented programming ii · queue queue is an adt data structure similar to stack,...
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CMSC 132: Object-Oriented Programming II
Stack and Queue
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Stack
Allows access to only the last item inserted.
An item is inserted or removed from the stack from one end called the �top� of the stack.
This mechanism is called Last-In-First-Out (LIFO).
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Stack Example
Empty Stack
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Stack Example
Stack
Push 10
10Top
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Stack Example
Stack
Push 10
10
Top
Push 20
20
Push 30
30
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Stack Example
Stack
Push 10
10Top
Push 20
20
Push 30
Pop
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Stack Example
Stack
Push 10
10
Top
Push 20
20
Push 30
Pop Push 10
10
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Applications
JVM stack machineFunction CallExpression EvaluationPushdown AutomataRecursive à Iterative Function Conversion
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Stack Interface
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public interface Stack<T> extends Iterable<T>{void push(T t);T pop();// throws EmptyStackException;T peek() ;//throws EmptyStackException; boolean isEmpty();int size();
}
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Implement Stack Using an Array
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public class ArrayStack<T> implements Stack<T> {private T[] items;private int N; // number of elements in the stackpublic ArrayStack(){
items = (T[])new Object[2];}
}
Stack Example
Empty Stack
N
4
3
2
1
0
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Pushing an item
N
4
3
2
1
0
Push 10
10
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Pushing more items
N
4
3
2
1
0 10
Push 10
Push 20
Push 30
20
30
What if the array
is full?13
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Removing an item
N
4
3
2
1
0 10
Push 10
Push 20
Push 30
20Pop
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Removing an item
N
4
3
2
1
0 10
Push 10
Push 20
Push 30
20Pop
Pop
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Array Implementation: push
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N
4
3
2
1
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public void push(T item){if(N == items.length){
resize(2 * items.length);} items[N++] = item;
}
Array Implementation: pop
Why?
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public T pop(){if(isEmpty()) throw new NoSuchElementException(); T item = items[--N];items[N] = null;return item;
}
Array Implementation: peek
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public T peek(){if(isEmpty())
throw new NoSuchElementException("Stack Underflow"); return items[N-1];
}
Implement Stack Using a Linked List
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public class LinkedStack<T> implements Stack<T> {private int N; private Node first;
private class Node{private T data;private Node next;Node(T item){
data = item;}
}
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Stack Example
Empty Stack
first = nullN =0
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Pushing an item
firstN =1
10
Push 10
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public void push(T item){Node old = first; first = new Node(item); first.next = old; N++;
}
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Pushing more items
firstN =3
30
Push 10
Push 20
Push 30
1020
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public void push(T item){Node old = first; first = new Node(item); first.next = old; N++;
}
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Pushing more items
firstN =2
20
Push 10
Push 20
Push 30
10
Pop
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public T pop(){if(isEmpty()){
throw new NoSuchElementException();} T item = first.data;first = first.next;N--;return item;
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Other methodsPeek
size
isEmpty
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public T peek(){if(isEmpty()){throw new NoSuchElementException();}return first.data;
}
public int size(){return N;
}
public boolean isEmpty(){return first == null;
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Testing the Stack
Size:77,7,6,6,5,5,4,4,3,3,2,2,1,1
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QueueQueue is an ADT data structure similar to stack, except that the first item to be inserted is the first one to be removed.
This mechanism is called First-In-First-Out (FIFO).
Placing an item in a queue: enqueue.
Removing an item from a queue: dequeue
Applications: printer queue, keystroke queue, etc.
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Queue
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public interface Queue<T> extends Iterable<T> {void enqueue(T t);T dequeue();// throws EmptyStackException; T peek() ;//throws EmptyStackException;boolean isEmpty();int size();
}
Queue Example
Empty Queue
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Adding an item
Empty Queue
enqueue 10
10
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Adding more items
Empty Queue
enqueue 10
30
enqueue 20
20
enqueue 30
10
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Adding more items
Empty Queue
enqueue 10
30
enqueue 20
20
enqueue 30dequeue
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Circular Array Implementation
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0 1 2 3 4 5
first last
N = 0
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Circular Array Implementation
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10 20 300 1 2 3 4 5
first last
enqueue 10enqueue 20enqueue 30
N =3
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Circular Array Implementation
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20 300 1 2 3 4 5
first last
enqueue 10enqueue 20enqueue 30
dequeue
N =2
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Circular Array Implementation
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20 30 40 500 1 2 3 4 5
first last
enqueue 10enqueue 20enqueue 30
dequeue
N =4
enqueue 40enqueue 50
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Circular Array Implementation
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20 30 40 50 600 1 2 3 4 5
firstlast
enqueue 10enqueue 20enqueue 30
dequeue
N =5
enqueue 40enqueue 50enqueue 60
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Circular Array Implementation
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30 40 50 60
0 1 2 3 4 5
firstlast
enqueue 10enqueue 20enqueue 30
dequeue
N =4
enqueue 40enqueue 50enqueue 60dequeue
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Implementing push (enqueue)
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30 40 50 600 1 2 3 4 5
firstlast
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public void enqueue(E item) {if (N == q.length) resize(2*q.length); q[last++] = item;if (last == q.length) last = 0; //wrap-around N++;
}
public E dequeue() {if(isEmpty()) throw
new NoSuchElementException("Queue underflow"); E item = q[first];q[first] = null;N--;first++;if (first == q.length) first = 0;// wrap-around
// shrink size of array if necessaryif (N > 0 && N == q.length/4)
resize(q.length/2); return item;
}
Implementing pop (dequeue)
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30 40 50 600 1 2 3 4 5
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Implementing resize
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70 30 40 50 60
0 1 2 3 4 5
30 40 50 60 70
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private void resize(int max){assert max >= N;E[] temp = (E[]) new Object[max]; for (int i = 0; i < N; i++) {
temp[i] = q[(first + i) % q.length];} q = temp;first = 0;last = N;
}
firstlast
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Linked List Queue
firstN =3
10 3020
last
• Add a new item• New node is added to tail (last.next)
• Remove a item• Remove the first node (first = first.next)
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