programming-examples/java/Data_Structures/LinkedQueue.java

 

/*************************************************************************
 *  Compilation:  javac LinkedQueue.java
 *  Execution:    java LinkedQueue < input.txt
 *  Dependencies: StdIn.java StdOut.java
 *  Data files:   http://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)
 *
 *************************************************************************/

import java.util.Iterator;
import java.util.NoSuchElementException;

import edu.princeton.cs.introcs.StdIn;
import edu.princeton.cs.introcs.StdOut;

/**
 *  The  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 <a href="http://algs4.cs.princeton.edu/13stacks">Section 1.3</a> of
 *   Algorithms, 4th Edition  by Robert Sedgewick and Kevin Wayne.
 *
 *  @author Robert Sedgewick
 *  @author Kevin Wayne
 */
public class LinkedQueue<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;
    }

    /**
     * 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
     */
    @Override
    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;
    } 
 

    /**
     * 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<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;
        }
    }


    /**
     * Unit tests the  LinkedQueue  data type.
     */
    public static void main(String[] args) {
        LinkedQueue<String> q = new LinkedQueue<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)");
    }
}