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Queue_Linked_List.java
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Queue_Linked_List.java
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/******************************************************************************
*
* A generic queue, implemented using a singly-linked list.
* Reference: http://algs4.cs.princeton.edu/13stacks/Queue_Linked_List.java.html
*
*
******************************************************************************/
import java.util.Iterator;
import java.util.NoSuchElementException;
/**
* 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 <em>enqueue</em>, <em>dequeue</em>, <em>peek</em>, <em>size</em>, and <em>is-empty</em>
* operations all take constant time in the worst case.
* <p>
*/
public class Queue_Linked_List<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 Queue_Linked_List() {
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<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;
}
}
public static void main(String[] args) {
Queue_Linked_List<Integer> queue = new Queue_Linked_List<Integer>();
queue.enqueue(1);
queue.enqueue(2);
queue.enqueue(3);
System.out.println("(" + queue.size() + " left on queue)");
System.out.println("Dequeued 1 item off the Queue");
queue.dequeue();
System.out.println("(" + queue.size() + " left on queue)");
System.out.println(queue.toString());
}
}
/* Output
(3 left on queue)
Dequeued 1 item off the Queue
(2 left on queue)
2 3
*/