public interface Queue<Item> extends Iterable<Item>
{
public void enqueue(Item item);
public Item dequeue();
public boolean isEmpty();
public int size();
}
Nothing...
/*************************************************************************
* Compilation: javac ResizingArrayQueue.java
* Execution: java ResizingArrayQueue < input.txt
* Data files: http://algs4.cs.princeton.edu/13stacks/tobe.txt
*
* Queue implementation with a resizing array.
*
* % java ResizingArrayQueue < tobe.txt
* to be or not to be (2 left on queue)
*
*************************************************************************/
import java.util.Iterator;
import java.util.NoSuchElementException;
public class ResizingArrayQueue<Item> implements Iterable<Item> {
private Item[] q; // queue elements
private int N = 0; // number of elements on queue
private int first = 0; // index of first element of queue
private int last = 0; // index of next available slot
// cast needed since no generic array creation in Java
public ResizingArrayQueue() {
q = (Item[]) new Object[2];
}
public boolean isEmpty() { return N == 0; }
public int size() { return N; }
// resize the underlying array
private void resize(int max) {
assert max >= N;
Item[] temp = (Item[]) new Object[max];
for (int i = 0; i < N; i++) {
temp[i] = q[(first + i) % q.length];
}
q = temp;
first = 0;
last = N;
}
public void enqueue(Item item) {
// double size of array if necessary and recopy to front of array
if (N == q.length) resize(2*q.length); // double size of array if necessary
q[last++] = item; // add item
if (last == q.length) last = 0; // wrap-around
N++;
}
// remove the least recently added item
public Item dequeue() {
if (isEmpty()) throw new NoSuchElementException("Queue underflow");
Item item = q[first];
q[first] = null; // to avoid loitering
N--;
first++;
if (first == q.length) first = 0; // wrap-around
// shrink size of array if necessary
if (N > 0 && N == q.length/4) resize(q.length/2);
return item;
}
public Iterator<Item> iterator() { return new ArrayIterator(); }
// an iterator, doesn't implement remove() since it's optional
private class ArrayIterator implements Iterator<Item> {
private int i = 0;
public boolean hasNext() { return i < N; }
public void remove() { throw new UnsupportedOperationException(); }
public Item next() {
if (!hasNext()) throw new NoSuchElementException();
Item item = q[(i + first) % q.length];
i++;
return item;
}
}
/**
* A test client.
*/
public static void main(String[] args) {
ResizingArrayQueue<String> q = new ResizingArrayQueue<String>();
while (!StdIn.isEmpty()) {
String item = StdIn.readString();
if (!item.equals("-")) q.enqueue(item);
else if (!q.isEmpty()) StdOut.print(q.dequeue() + " ");
}
StdOut.println("(" + q.size() + " left on queue)");
}
}
To enqueue() an item, we add it to the end of the list; to dequeue() an item, we remove it from the beginning of the list.
/*************************************************************************
* Compilation: javac Queue.java
* Execution: java Queue < input.txt
* Data files: http://algs4.cs.princeton.edu/13stacks/tobe.txt
*
* A generic queue, implemented using a linked list.
*
* % java Queue < tobe.txt
* to be or not to be (2 left on queue)
*
*************************************************************************/
import java.util.Iterator;
import java.util.NoSuchElementException;
/**
* The <tt>Queue</tt> class represents a first-in-first-out (FIFO)
* queue of generic items.
* It supports the usual <em>enqueue</em> and <em>dequeue</em>
* operations, along with methods for peeking at the top item,
* testing if the queue is empty, and iterating through
* the items in FIFO order.
* <p>
* All queue operations except iteration are constant time.
* <p>
* For additional documentation, see <a href="http://algs4.cs.princeton.edu/13stacks">Section 1.3</a> of
* <i>Algorithms, 4th Edition</i> by Robert Sedgewick and Kevin Wayne.
*/
public class Queue<Item> implements Iterable<Item> {
private int N; // number of elements on queue
private Node first; // beginning of queue
private Node last; // end of queue
// helper linked list class
private class Node {
private Item item;
private Node next;
}
/**
* Create an empty queue.
*/
public Queue() {
first = null;
last = null;
N = 0;
assert check();
}
/**
* Is the queue empty?
*/
public boolean isEmpty() {
return first == null;
}
/**
* Return the number of items in the queue.
*/
public int size() {
return N;
}
/**
* Return the item least recently added to the queue.
* @throws java.util.NoSuchElementException if queue is empty.
*/
public Item peek() {
if (isEmpty()) throw new NoSuchElementException("Queue underflow");
return first.item;
}
/**
* Add the item to the queue.
*/
public void enqueue(Item item) {
Node oldlast = last;
last = new Node();
last.item = item;
last.next = null;
if (isEmpty()) first = last;
else oldlast.next = last;
N++;
assert check();
}
/**
* Remove and return the item on the queue least recently added.
* @throws java.util.NoSuchElementException if queue is empty.
*/
public Item dequeue() {
if (isEmpty()) throw new NoSuchElementException("Queue underflow");
Item item = first.item;
first = first.next;
N--;
if (isEmpty()) last = null; // to avoid loitering
assert check();
return item;
}
/**
* Return string representation.
*/
public String toString() {
StringBuilder s = new StringBuilder();
for (Item item : this)
s.append(item + " ");
return s.toString();
}
// check internal invariants
private boolean check() {
if (N == 0) {
if (first != null) return false;
if (last != null) return false;
}
else if (N == 1) {
if (first == null || last == null) return false;
if (first != last) return false;
if (first.next != null) return false;
}
else {
if (first == last) return false;
if (first.next == null) return false;
if (last.next != null) return false;
// check internal consistency of instance variable N
int numberOfNodes = 0;
for (Node x = first; x != null; x = x.next) {
numberOfNodes++;
}
if (numberOfNodes != N) return false;
// check internal consistency of instance variable last
Node lastNode = first;
while (lastNode.next != null) {
lastNode = lastNode.next;
}
if (last != lastNode) return false;
}
return true;
}
/**
* Return an iterator that iterates over the items on the queue in FIFO order.
*/
public Iterator<Item> iterator() {
return new ListIterator();
}
// an iterator, doesn't implement remove() since it's optional
private class ListIterator implements Iterator<Item> {
private Node current = first;
public boolean hasNext() { return current != null; }
public void remove() { throw new UnsupportedOperationException(); }
public Item next() {
if (!hasNext()) throw new NoSuchElementException();
Item item = current.item;
current = current.next;
return item;
}
}
/**
* A test client.
*/
public static void main(String[] args) {
Queue<String> q = new Queue<String>();
while (!StdIn.isEmpty()) {
String item = StdIn.readString();
if (!item.equals("-")) q.enqueue(item);
else if (!q.isEmpty()) StdOut.print(q.dequeue() + " ");
}
StdOut.println("(" + q.size() + " left on queue)");
}
}