edu.princeton.cs.algs4.LinkedQueue Maven / Gradle / Ivy
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/******************************************************************************
* Compilation: javac LinkedQueue.java
* Execution: java LinkedQueue < input.txt
* Dependencies: StdIn.java StdOut.java
* Data files: https://algs4.cs.princeton.edu/13stacks/tobe.txt
*
* A generic queue, implemented using a singly linked list.
*
* % java Queue < tobe.txt
* to be or not to be (2 left on queue)
*
******************************************************************************/
package edu.princeton.cs.algs4;
import java.util.Iterator;
import java.util.NoSuchElementException;
/**
* The {@code LinkedQueue} class represents a first-in-first-out (FIFO)
* queue of generic items.
* It supports the usual enqueue and dequeue
* operations, along with methods for peeking at the first item,
* testing if the queue is empty, and iterating through
* the items in FIFO order.
*
* This implementation uses a singly linked list with a non-static nested class
* for linked-list nodes. See {@link Queue} for a version that uses a static nested class.
* The enqueue, dequeue, peek, size, and is-empty
* operations all take constant time in the worst case.
*
* For additional documentation, see Section 1.3 of
* Algorithms, 4th Edition by Robert Sedgewick and Kevin Wayne.
*
* @author Robert Sedgewick
* @author Kevin Wayne
*/
public class LinkedQueue- implements Iterable
- {
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;
}
/**
* Initializes an empty queue.
*/
public LinkedQueue() {
first = null;
last = null;
n = 0;
assert check();
}
/**
* Is this queue empty?
* @return true if this queue is empty; false otherwise
*/
public boolean isEmpty() {
return first == null;
}
/**
* Returns the number of items in this queue.
* @return the number of items in this queue
*/
public int size() {
return n;
}
/**
* Returns the item least recently added to this queue.
* @return the item least recently added to this queue
* @throws java.util.NoSuchElementException if this queue is empty
*/
public Item peek() {
if (isEmpty()) throw new NoSuchElementException("Queue underflow");
return first.item;
}
/**
* Adds the item to this queue.
* @param item the item to add
*/
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();
}
/**
* Removes and returns the item on this queue that was least recently added.
* @return the item on this queue that was least recently added
* @throws java.util.NoSuchElementException if this 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;
}
/**
* Returns a string representation of this queue.
* @return the sequence of items in FIFO order, separated by spaces
*/
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) {
return false;
}
else 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 == null || last == null) return false;
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 && numberOfNodes <= n; 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;
}
/**
* Returns an iterator that iterates over the items in this queue in FIFO order.
* @return an iterator that iterates over the items in this queue in FIFO order
*/
public Iterator
- iterator() {
return new ListIterator();
}
// an iterator, doesn't implement remove() since it's optional
private class ListIterator implements Iterator
- {
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;
}
}
/**
* Unit tests the {@code LinkedQueue} data type.
*
* @param args the command-line arguments
*/
public static void main(String[] args) {
LinkedQueue
queue = new LinkedQueue();
while (!StdIn.isEmpty()) {
String item = StdIn.readString();
if (!item.equals("-"))
queue.enqueue(item);
else if (!queue.isEmpty())
StdOut.print(queue.dequeue() + " ");
}
StdOut.println("(" + queue.size() + " left on queue)");
}
}
/******************************************************************************
* Copyright 2002-2018, Robert Sedgewick and Kevin Wayne.
*
* This file is part of algs4.jar, which accompanies the textbook
*
* Algorithms, 4th edition by Robert Sedgewick and Kevin Wayne,
* Addison-Wesley Professional, 2011, ISBN 0-321-57351-X.
* http://algs4.cs.princeton.edu
*
*
* algs4.jar is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
*
* algs4.jar is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with algs4.jar. If not, see http://www.gnu.org/licenses.
******************************************************************************/