Java新手问题：我试图在Java中实现deque并且遇到dequeueBack问题（从队列后面删除一个元素）和enqueueFront（在队列前面添加一个元素）方法。我已经得到了相反的工作方法（dequeueFront和enqueueBack），但我现在很难过。有什么建议吗？
public class Carr_A06Q4
{
/**
* Program entry point for deque testing.
* @param args Argument list.
*/
public static void main(String[] args)
{
LinkedDequeue<Integer> deque = new LinkedDequeue<Integer>();
System.out.println("DEQUE TESTING");
//per Q1
deque.enqueueBack(3);
deque.enqueueBack(7);
deque.enqueueBack(4);
deque.dequeueFront();
deque.enqueueBack(9);
deque.enqueueBack(8);
deque.dequeueFront();
System.out.println("The size of the deque is: " + deque.size());
System.out.println("The deque contains:\n" + deque.toString());
//new features
System.out.println(deque.dequeueFront());
deque.enqueueFront(1);
deque.enqueueFront(11);
deque.enqueueFront(3);
deque.enqueueFront(5);
System.out.println(deque.dequeueBack());
System.out.println(deque.dequeueBack());
System.out.println(deque.last());
deque.dequeueFront();
deque.dequeueFront();
System.out.println(deque.first());
System.out.println("The size of the deque is: " + deque.size());
System.out.println("The deque contains:\n" + deque.toString());
}
/**
* LinkedDeque represents a linked implementation of a deque.
*
*/
public static class LinkedDequeue<T> implements DequeADT<T>
{
private int count;
private LinearDoubleNode<T> head, tail; //front, back
/**
* Creates an empty queue.
*/
public LinkedDequeue()
{
count = 0;
head = tail = null;
}
/**
* Adds the specified element to the tail of this queue.
* @param element the element to be added to the tail of the queue
*/
public void enqueueBack(T element)
{
LinearDoubleNode<T> node = new LinearDoubleNode<T>(element);
if (isEmpty())
head = node;
else
tail.setNext(node);
tail = node;
count++;
}
/**
* Adds the specified element to the head of this queue.
* @param element the element to be added to the head of the queue
*/
public void enqueueFront(T element)
{
LinearDoubleNode<T> node = new LinearDoubleNode<T>(element);
count++ ;
if (head == null)
{
head = node;
}
else
{
node.setNext(head);
head = node;
}
}
/**
* Removes the element at the head of this queue and returns a
* reference to it.
* @return the element at the head of this queue
* @throws EmptyCollectionException if the queue is empty
*/
public T dequeueFront() throws EmptyCollectionException
{
if (isEmpty())
throw new EmptyCollectionException("queue");
T result = head.getElement();
head = head.getNext();
count--;
if (isEmpty())
head = null;
return result;
}
/**
* Removes the element at the tail of this queue and returns a
* reference to it.
* @return the element at the tail of this queue
* @throws EmptyCollectionException if the queue is empty
*/
public T dequeueBack() throws EmptyCollectionException
{
if (isEmpty())
throw new EmptyCollectionException("queue");
T result = tail.getElement();
tail = tail.getNext();
if (isEmpty())
head = null;
count --;
return result;
}
/**
* Returns a reference to the element at the head of this queue.
* The element is not removed from the queue.
* @return a reference to the first element in this queue
* @throws EmptyCollectionsException if the queue is empty
*/
public T first() throws EmptyCollectionException
{
if (isEmpty())
throw new EmptyCollectionException("stack");
T result = head.getElement();
return result;
}
/**
* Returns a reference to the element at the tail of this queue.
* The element is not removed from the queue.
* @return a reference to the first element in this queue
* @throws EmptyCollectionsException if the queue is empty
*/
public T last() throws EmptyCollectionException
{
if (isEmpty())
throw new EmptyCollectionException("stack");
T result = tail.getElement();
return result;
}
/**
* Returns true if this queue is empty and false otherwise.
* @return true if this queue is empty
*/
public boolean isEmpty()
{
if (head == null)
{
return true;
}
else
{
return false;
}
}
/**
* Returns the number of elements currently in this queue.
* @return the number of elements in the queue
*/
public int size()
{
return count;
}
/**
* Returns a string representation of this queue. The front element
* occurs first, and each element is separated by a space. If the
* queue is empty, returns "empty".
* @return the string representation of the queue
*/
public String toString()
{
StringBuilder sb = new StringBuilder();
LinearDoubleNode<T> tmp = head;
while (tmp != null)
{
sb.append(tmp.getElement()).append(" ");
tmp = tmp.getNext();
}
return sb.toString();
}
}
}
我需要输出：
DEQUE TESTING： 双端队列的大小是：3 双端队列包含：
498
4
8
9
1
11
双端队列的大小是：2 双端队列包含： 11 1
但我得到的是
DEQUE TESTING： 双端队列的大小是：3 双端队列包含：
498
4
8
然后我在dequeueBack方法中得到一个NullPointerException。
感谢任何帮助！
此外，界面是：
public interface DequeADT<T>
{
/**
* Adds one element to the front of this deque.
* @param element the element to be added to the front of the deque
*/
public void enqueueFront(T element); //deque specific
/**
* Adds one element to the back of this deque.
* @param element the element to be added to the back of the deque
*/
public void enqueueBack(T element);
/**
* Removes and returns the element at the front of this deque.
* Should throw an exception if the deque is empty.
* @return the element at the front of this deque
*/
public T dequeueFront();
/**
* Removes and returns the element at the back of this deque.
* Should throw an exception if the deque is empty.
* @return the element at the back of the deque.
*/
public T dequeueBack(); //deque specific
/**
* Returns, without removing, the element at the front of this deque.
* Should throw an exception if the deque is empty.
* @return the first element in the deque
*/
public T first();
/**
* Returns, without removing, the element at the back of this deque.
* Should throw an exception if the deque is empty.
* @return the last element in the deque
*/
public T last(); //deque specific
/**
* Returns true if this deque is empty and false otherwise.
* @return true if this deque is empty
*/
public boolean isEmpty();
/**
* Returns the number of elements in this deque.
* @return the number of elements in the deque
*/
public int size();
/**
* Returns a string representation of this deque. The front element
* occurs first, and each element is separated by a space. If the
* deque is empty, returns "empty".
* @return the string representation of the deque
*/
public String toString();
}
并且节点的描述是：
public class LinearDoubleNode<T>
{
private LinearDoubleNode<T> next;
private T element;
/**
* Creates an empty node.
*/
public LinearDoubleNode()
{
next = null;
element = null;
}
/**
* Creates a node storing the specified element.
* @param elem element to be stored
*/
public LinearDoubleNode(T elem)
{
next = null;
element = elem;
}
/**
* Returns the node that follows this one.
* @return reference to next node
*/
public LinearDoubleNode<T> getNext()
{
return next;
}
/**
* Sets the node that follows this one.
* @param node node to follow this one
*/
public void setNext(LinearDoubleNode<T> node)
{
next = node;
}
/**
* Returns the element stored in this node.
* @return element stored at the node
*/
public T getElement()
{
return element;
}
/**
* Sets the element stored in this node.
* @param elem element to be stored at this node
*/
public void setElement(T elem)
{
element = elem;
}
}
答案 0 :(得分：1)
您可以执行此操作的唯一方法是将内部列表设为double-linked。
否则，您必须扫描整个列表以查找倒数第二个节点，以便删除最后一个节点。
Double-ended queue➔Double-linked list
或动态数组（见Implementations部分）。
答案 1 :(得分：0)
public class LinkedDQueue实现了DQueue {
private int size;
private LinearDoubleNode<T> head, tail; // front, back
private int capcity;
public LinkedDQueue() {
capcity = 10;
}
public LinkedDQueue(int capcity) {
this.capcity = capcity;
}
@Override
public boolean full() {
return size == capcity;
}
@Override
public void addFirst(T e) throws CapcityExceededException {
if (full())
throw new CapcityExceededException("size overflow");
LinearDoubleNode<T> node = new LinearDoubleNode<T>(e);
if (head == null) {
head = node;
tail = node;
} else {
node.setNext(head);
head = node;
}
size++;
}
@Override
public void addLast(T e) throws CapcityExceededException {
LinearDoubleNode<T> node = new LinearDoubleNode<T>(e);
if (full())
throw new CapcityExceededException("size overflow");
if (tail == null) {
head = tail = node;
} else {
tail.setNext(node);
node.setPrevious(tail);
}
tail = node;
size++;
}
@Override
public T removeFirst() {
if (isEmpty())
return null;
T result = head.getElement();
head.getNext().setPrevious(null);
head = head.getNext();
size--;
return result;
}
@Override
public T removeLast() {
LinearDoubleNode<T> temp = tail; // Save address of Node to delete
if (isEmpty()) {
return null;
}
if (head == tail) {
head = null;
tail = null;
size = 0;
return tail.getElement();
}
T result = tail.getElement();
tail = temp.getPrevious();
tail.setNext(null);
size--;
return result;
}
@Override
public T getFirst() {
if (isEmpty())
return null;
T result = head.getElement();
return result;
}
@Override
public T getLast() {
if (isEmpty())
return null;
T result = tail.getElement();
return result;
}
@Override
public int length() {
return size;
}
@Override
public void reverse() {
}
@Override
public boolean isEmpty() {
if (head == null) {
return true;
} else {
return false;
}
}
@Override
public int size() {
return size;
}
@Override
public String toString() {
LinearDoubleNode<T> temp = head; // Save address of Node to delete
StringBuilder builder = new StringBuilder();
while (temp != null) {
builder.append(temp.getElement() + "");
temp = temp.getNext();
}
return builder.toString();
}
class LinearDoubleNode<T> {
private LinearDoubleNode<T> next;
private LinearDoubleNode<T> previous;
private T element;
/**
* Creates an empty node.
*/
public LinearDoubleNode() {
next = null;
previous = null;
element = null;
}
/**
* Creates a node storing the specified element.
*
* @param elem
* element to be stored
*/
public LinearDoubleNode(T elem) {
next = null;
previous = null;
element = elem;
}
/**
* Returns the node that follows this one.
*
* @return reference to next node
*/
public LinearDoubleNode<T> getNext() {
return next;
}
public LinearDoubleNode<T> getPrevious() {
return previous;
}
public void setPrevious(LinearDoubleNode<T> previous) {
this.previous = previous;
}
/**
* Sets the node that follows this one.
*
* @param node
* node to follow this one
*/
public void setNext(LinearDoubleNode<T> node) {
next = node;
}
/**
* Returns the element stored in this node.
*
* @return element stored at the node
*/
public T getElement() {
return element;
}
/**
* Sets the element stored in this node.
*
* @param elem
* element to be stored at this node
*/
public void setElement(T elem) {
element = elem;
}
}
}
class CapcityExceededException extends Exception { 私有字符串消息;
public CapcityExceededException(String message) {
super(message);
this.message = message;
}
}
答案 2 :(得分：0)
import java.util.Iterator;
import java.util.NoSuchElementException;
/**
* The type Deque.
*
* @param <Item> the type parameter
*/
public class Deque<Item> implements Iterable<Item> {
/**
* The Head.
*/
private Node<Item> head;
/**
* The Tail.
*/
private Node<Item> tail;
/**
* The Deque size.
*/
private int dequeSize;
private class Node<Item> {
/**
* The Data.
*/
Item data;
/**
* The Next.
*/
Node<Item> next;
/**
* The Prev.
*/
Node<Item> prev;
/**
* Instantiates a new Node.
*
* @param data the data
*/
Node(Item data) {
this.data = data;
}
}
/**
* Instantiates a new Deque.
*/
public Deque() {
dequeSize = 0;
}
/**
* Is empty boolean.
*
* @return the boolean
*/
public boolean isEmpty() {
return dequeSize == 0;
}
/**
* Size int.
*
* @return the int
*/
public int size() {
return dequeSize;
}
/**
* Add first.
*
* @param item the item
*/
public void addFirst(Item item) {
if (item == null) {
throw new IllegalArgumentException();
}
Node<Item> newNode = new Node<Item>(item);
if (head == null) {
head = newNode;
tail = newNode;
} else {
head.prev = newNode;
newNode.next = head;
head = newNode;
}
dequeSize++;
}
/**
* Add last.
*
* @param item the item
*/
public void addLast(Item item) {
if (item == null) {
throw new IllegalArgumentException();
}
Node<Item> newNode = new Node<Item>(item);
if (head == null) {
head = newNode;
tail = newNode;
} else {
tail.next = newNode;
newNode.prev = tail;
tail = newNode;
}
dequeSize++;
}
/**
* Remove first item.
*
* @return the item
*/
public Item removeFirst() {
if (isEmpty()) {
throw new NoSuchElementException();
}
Item headData = head.data;
if (dequeSize == 1) {
head = null;
tail = null;
} else {
Node<Item> headNext = head.next;
headNext.prev = null;
head = headNext;
}
dequeSize--;
return headData;
}
/**
* Remove last item.
*
* @return the item
*/
public Item removeLast() {
if (isEmpty()) {
throw new NoSuchElementException();
}
Item tailData = tail.data;
if (dequeSize == 1) {
head = null;
tail = null;
} else {
Node<Item> tailPrev = tail.prev;
tailPrev.next = null;
tail = tailPrev;
}
dequeSize--;
return tailData;
}
/**
* Iterator iterator.
*
* @return the iterator
*/
public Iterator<Item> iterator() {
return new CustomIterator();
}
private class CustomIterator implements Iterator<Item> {
private Node<Item> temp;
/**
* Instantiates a new Custom iterator.
*/
CustomIterator() {
temp = head;
}
public boolean hasNext() {
return temp.next != null;
}
public Item next() {
if (!hasNext()) {
throw new NoSuchElementException();
}
Item tempData = temp.data;
temp = temp.next;
return tempData;
}
public void remove() {
throw new UnsupportedOperationException();
}
}
/**
* The entry point of application.
*
* @param args the input arguments
*/
public static void main(String[] args) {
// unit testing (required)
}
}