/*

  * Copyright (c) 1997, 2011, Oracle and/or its affiliates. All rights reserved.

  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

  *

  * This code is free software; you can redistribute it and/or modify it

  * under the terms of the GNU General Public License version 2 only, as

  * published by the Free Software Foundation.  Oracle designates this

  * particular file as subject to the "Classpath" exception as provided

  * by Oracle in the LICENSE file that accompanied this code.

  *

  * This code 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

  * version 2 for more details (a copy is included in the LICENSE file that

  * accompanied this code).

  *

  * You should have received a copy of the GNU General Public License version

  * 2 along with this work; if not, write to the Free Software Foundation,

  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

  *

  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

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  * questions.

  */

 package java.util;

 /**

  * Doubly-linked list implementation of the {@code List} and {@code Deque}

  * interfaces.  Implements all optional list operations, and permits all

  * elements (including {@code null}).

  *

  * <p>All of the operations perform as could be expected for a doubly-linked

  * list.  Operations that index into the list will traverse the list from

  * the beginning or the end, whichever is closer to the specified index.

  *

  * <p><strong>Note that this implementation is not synchronized.</strong>

  * If multiple threads access a linked list concurrently, and at least

  * one of the threads modifies the list structurally, it <i>must</i> be

  * synchronized externally.  (A structural modification is any operation

  * that adds or deletes one or more elements; merely setting the value of

  * an element is not a structural modification.)  This is typically

  * accomplished by synchronizing on some object that naturally

  * encapsulates the list.

  *

  * If no such object exists, the list should be "wrapped" using the

  * {@link Collections#synchronizedList Collections.synchronizedList}

  * method.  This is best done at creation time, to prevent accidental

  * unsynchronized access to the list:<pre>

  *   List list = Collections.synchronizedList(new LinkedList(...));</pre>

  *

  * <p>The iterators returned by this class's {@code iterator} and

  * {@code listIterator} methods are <i>fail-fast</i>: if the list is

  * structurally modified at any time after the iterator is created, in

  * any way except through the Iterator's own {@code remove} or

  * {@code add} methods, the iterator will throw a {@link

  * ConcurrentModificationException}.  Thus, in the face of concurrent

  * modification, the iterator fails quickly and cleanly, rather than

  * risking arbitrary, non-deterministic behavior at an undetermined

  * time in the future.

  *

  * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed

  * as it is, generally speaking, impossible to make any hard guarantees in the

  * presence of unsynchronized concurrent modification.  Fail-fast iterators

  * throw {@code ConcurrentModificationException} on a best-effort basis.

  * Therefore, it would be wrong to write a program that depended on this

  * exception for its correctness:   <i>the fail-fast behavior of iterators

  * should be used only to detect bugs.</i>

  *

  * <p>This class is a member of the

  * <a href="{@docRoot}/../technotes/guides/collections/index.html">

  * Java Collections Framework</a>.

  *

  * @author  Josh Bloch

  * @see     List

  * @see     ArrayList

  * @since 1.2

  * @param <E> the type of elements held in this collection

  */

 public class LinkedList<E>

     extends AbstractSequentialList<E>

     implements List<E>, Deque<E>, Cloneable, java.io.Serializable

 {

     transient int size = 0;

     /**

      * Pointer to first node.

      * Invariant: (first == null && last == null) ||

      *            (first.prev == null && first.item != null)

      */

     transient Node<E> first;

     /**

      * Pointer to last node.

      * Invariant: (first == null && last == null) ||

      *            (last.next == null && last.item != null)

      */

     transient Node<E> last;

     /**

      * Constructs an empty list.

      */

     public LinkedList() {

     }

     /**

      * Constructs a list containing the elements of the specified

      * collection, in the order they are returned by the collection's

      * iterator.

      *

      * @param  c the collection whose elements are to be placed into this list

      * @throws NullPointerException if the specified collection is null

      */

     public LinkedList(Collection<? extends E> c) {

         this();

         addAll(c);

     }

     /**

      * Links e as first element.

      */

     private void linkFirst(E e) {

         final Node<E> f = first;

         final Node<E> newNode = new Node<>(null, e, f);

         first = newNode;

         if (f == null)

             last = newNode;

         else

             f.prev = newNode;

         size++;

         modCount++;

     }

     /**

      * Links e as last element.

      */

     void linkLast(E e) {

         final Node<E> l = last;

         final Node<E> newNode = new Node<>(l, e, null);

         last = newNode;

         if (l == null)

             first = newNode;

         else

             l.next = newNode;

         size++;

         modCount++;

     }

     /**

      * Inserts element e before non-null Node succ.

      */

     void linkBefore(E e, Node<E> succ) {

         // assert succ != null;

         final Node<E> pred = succ.prev;

         final Node<E> newNode = new Node<>(pred, e, succ);

         succ.prev = newNode;

         if (pred == null)

             first = newNode;

         else

             pred.next = newNode;

         size++;

         modCount++;

     }

     /**

      * Unlinks non-null first node f.

      */

     private E unlinkFirst(Node<E> f) {

         // assert f == first && f != null;

         final E element = f.item;

         final Node<E> next = f.next;

         f.item = null;

         f.next = null; // help GC

         first = next;

         if (next == null)

             last = null;

         else

             next.prev = null;

         size--;

         modCount++;

         return element;

     }

     /**

      * Unlinks non-null last node l.

      */

     private E unlinkLast(Node<E> l) {

         // assert l == last && l != null;

         final E element = l.item;

         final Node<E> prev = l.prev;

         l.item = null;

         l.prev = null; // help GC

         last = prev;

         if (prev == null)

             first = null;

         else

             prev.next = null;

         size--;

         modCount++;

         return element;

     }

     /**

      * Unlinks non-null node x.

      */

     E unlink(Node<E> x) {

         // assert x != null;

         final E element = x.item;

         final Node<E> next = x.next;

         final Node<E> prev = x.prev;

         if (prev == null) {

             first = next;

         } else {

             prev.next = next;

             x.prev = null;

         }

         if (next == null) {

             last = prev;

         } else {

             next.prev = prev;

             x.next = null;

         }

         x.item = null;

         size--;

         modCount++;

         return element;

     }

     /**

      * Returns the first element in this list.

      *

      * @return the first element in this list

      * @throws NoSuchElementException if this list is empty

      */

     public E getFirst() {

         final Node<E> f = first;

         if (f == null)

             throw new NoSuchElementException();

         return f.item;

     }

     /**

      * Returns the last element in this list.

      *

      * @return the last element in this list

      * @throws NoSuchElementException if this list is empty

      */

     public E getLast() {

         final Node<E> l = last;

         if (l == null)

             throw new NoSuchElementException();

         return l.item;

     }

     /**

      * Removes and returns the first element from this list.

      *

      * @return the first element from this list

      * @throws NoSuchElementException if this list is empty

      */

     public E removeFirst() {

         final Node<E> f = first;

         if (f == null)

             throw new NoSuchElementException();

         return unlinkFirst(f);

     }

     /**

      * Removes and returns the last element from this list.

      *

      * @return the last element from this list

      * @throws NoSuchElementException if this list is empty

      */

     public E removeLast() {

         final Node<E> l = last;

         if (l == null)

             throw new NoSuchElementException();

         return unlinkLast(l);

     }

     /**

      * Inserts the specified element at the beginning of this list.

      *

      * @param e the element to add

      */

     public void addFirst(E e) {

         linkFirst(e);

     }

     /**

      * Appends the specified element to the end of this list.

      *

      * <p>This method is equivalent to {@link #add}.

      *

      * @param e the element to add

      */

     public void addLast(E e) {

         linkLast(e);

     }

     /**

      * Returns {@code true} if this list contains the specified element.

      * More formally, returns {@code true} if and only if this list contains

      * at least one element {@code e} such that

      * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.

      *

      * @param o element whose presence in this list is to be tested

      * @return {@code true} if this list contains the specified element

      */

     public boolean contains(Object o) {

         return indexOf(o) != -1;

     }

     /**

      * Returns the number of elements in this list.

      *

      * @return the number of elements in this list

      */

     public int size() {

         return size;

     }

     /**

      * Appends the specified element to the end of this list.

      *

      * <p>This method is equivalent to {@link #addLast}.

      *

      * @param e element to be appended to this list

      * @return {@code true} (as specified by {@link Collection#add})

      */

     public boolean add(E e) {

         linkLast(e);

         return true;

     }

     /**

      * Removes the first occurrence of the specified element from this list,

      * if it is present.  If this list does not contain the element, it is

      * unchanged.  More formally, removes the element with the lowest index

      * {@code i} such that

      * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>

      * (if such an element exists).  Returns {@code true} if this list

      * contained the specified element (or equivalently, if this list

      * changed as a result of the call).

      *

      * @param o element to be removed from this list, if present

      * @return {@code true} if this list contained the specified element

      */

     public boolean remove(Object o) {

         if (o == null) {

             for (Node<E> x = first; x != null; x = x.next) {

                 if (x.item == null) {

                     unlink(x);

                     return true;

                 }

             }

         } else {

             for (Node<E> x = first; x != null; x = x.next) {

                 if (o.equals(x.item)) {

                     unlink(x);

                     return true;

                 }

             }

         }

         return false;

     }

     /**

      * Appends all of the elements in the specified collection to the end of

      * this list, in the order that they are returned by the specified

      * collection's iterator.  The behavior of this operation is undefined if

      * the specified collection is modified while the operation is in

      * progress.  (Note that this will occur if the specified collection is

      * this list, and it's nonempty.)

      *

      * @param c collection containing elements to be added to this list

      * @return {@code true} if this list changed as a result of the call

      * @throws NullPointerException if the specified collection is null

      */

     public boolean addAll(Collection<? extends E> c) {

         return addAll(size, c);

     }

     /**

      * Inserts all of the elements in the specified collection into this

      * list, starting at the specified position.  Shifts the element

      * currently at that position (if any) and any subsequent elements to

      * the right (increases their indices).  The new elements will appear

      * in the list in the order that they are returned by the

      * specified collection's iterator.

      *

      * @param index index at which to insert the first element

      *              from the specified collection

      * @param c collection containing elements to be added to this list

      * @return {@code true} if this list changed as a result of the call

      * @throws IndexOutOfBoundsException {@inheritDoc}

      * @throws NullPointerException if the specified collection is null

      */

     public boolean addAll(int index, Collection<? extends E> c) {

         checkPositionIndex(index);

         Object[] a = c.toArray();

         int numNew = a.length;

         if (numNew == 0)

             return false;

         Node<E> pred, succ;

         if (index == size) {

             succ = null;

             pred = last;

         } else {

             succ = node(index);

             pred = succ.prev;

         }

         for (Object o : a) {

             @SuppressWarnings("unchecked") E e = (E) o;

             Node<E> newNode = new Node<>(pred, e, null);

             if (pred == null)

                 first = newNode;

             else

                 pred.next = newNode;

             pred = newNode;

         }

         if (succ == null) {

             last = pred;

         } else {

             pred.next = succ;

             succ.prev = pred;

         }

         size += numNew;

         modCount++;

         return true;

     }

     /**

      * Removes all of the elements from this list.

      * The list will be empty after this call returns.

      */

     public void clear() {

         // Clearing all of the links between nodes is "unnecessary", but:

         // - helps a generational GC if the discarded nodes inhabit

         //   more than one generation

         // - is sure to free memory even if there is a reachable Iterator

         for (Node<E> x = first; x != null; ) {

             Node<E> next = x.next;

             x.item = null;

             x.next = null;

             x.prev = null;

             x = next;

         }

         first = last = null;

         size = 0;

         modCount++;

     }

     // Positional Access Operations

     /**

      * Returns the element at the specified position in this list.

      *

      * @param index index of the element to return

      * @return the element at the specified position in this list

      * @throws IndexOutOfBoundsException {@inheritDoc}

      */

     public E get(int index) {

         checkElementIndex(index);

         return node(index).item;

     }

     /**

      * Replaces the element at the specified position in this list with the

      * specified element.

      *

      * @param index index of the element to replace

      * @param element element to be stored at the specified position

      * @return the element previously at the specified position

      * @throws IndexOutOfBoundsException {@inheritDoc}

      */

     public E set(int index, E element) {

         checkElementIndex(index);

         Node<E> x = node(index);

         E oldVal = x.item;

         x.item = element;

         return oldVal;

     }

     /**

      * Inserts the specified element at the specified position in this list.

      * Shifts the element currently at that position (if any) and any

      * subsequent elements to the right (adds one to their indices).

      *

      * @param index index at which the specified element is to be inserted

      * @param element element to be inserted

      * @throws IndexOutOfBoundsException {@inheritDoc}

      */

     public void add(int index, E element) {

         checkPositionIndex(index);

         if (index == size)

             linkLast(element);

         else

             linkBefore(element, node(index));

     }

     /**

      * Removes the element at the specified position in this list.  Shifts any

      * subsequent elements to the left (subtracts one from their indices).

      * Returns the element that was removed from the list.

      *

      * @param index the index of the element to be removed

      * @return the element previously at the specified position

      * @throws IndexOutOfBoundsException {@inheritDoc}

      */

     public E remove(int index) {

         checkElementIndex(index);

         return unlink(node(index));

     }

     /**

      * Tells if the argument is the index of an existing element.

      */

     private boolean isElementIndex(int index) {

         return index >= 0 && index < size;

     }

     /**

      * Tells if the argument is the index of a valid position for an

      * iterator or an add operation.

      */

     private boolean isPositionIndex(int index) {

         return index >= 0 && index <= size;

     }

     /**

      * Constructs an IndexOutOfBoundsException detail message.

      * Of the many possible refactorings of the error handling code,

      * this "outlining" performs best with both server and client VMs.

      */

     private String outOfBoundsMsg(int index) {

         return "Index: "+index+", Size: "+size;

     }

     private void checkElementIndex(int index) {

         if (!isElementIndex(index))

             throw new IndexOutOfBoundsException(outOfBoundsMsg(index));

     }

     private void checkPositionIndex(int index) {

         if (!isPositionIndex(index))

             throw new IndexOutOfBoundsException(outOfBoundsMsg(index));

     }

     /**

      * Returns the (non-null) Node at the specified element index.

      */

     Node<E> node(int index) {

         // assert isElementIndex(index);

         if (index < (size >> 1)) {

             Node<E> x = first;

             for (int i = 0; i < index; i++)

                 x = x.next;

             return x;

         } else {

             Node<E> x = last;

             for (int i = size - 1; i > index; i--)

                 x = x.prev;

             return x;

         }

     }

     // Search Operations

     /**

      * Returns the index of the first occurrence of the specified element

      * in this list, or -1 if this list does not contain the element.

      * More formally, returns the lowest index {@code i} such that

      * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,

      * or -1 if there is no such index.

      *

      * @param o element to search for

      * @return the index of the first occurrence of the specified element in

      *         this list, or -1 if this list does not contain the element

      */

     public int indexOf(Object o) {

         int index = 0;

         if (o == null) {

             for (Node<E> x = first; x != null; x = x.next) {

                 if (x.item == null)

                     return index;

                 index++;

             }

         } else {

             for (Node<E> x = first; x != null; x = x.next) {

                 if (o.equals(x.item))

                     return index;

                 index++;

             }

         }

         return -1;

     }

     /**

      * Returns the index of the last occurrence of the specified element

      * in this list, or -1 if this list does not contain the element.

      * More formally, returns the highest index {@code i} such that

      * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,

      * or -1 if there is no such index.

      *

      * @param o element to search for

      * @return the index of the last occurrence of the specified element in

      *         this list, or -1 if this list does not contain the element

      */

     public int lastIndexOf(Object o) {

         int index = size;

         if (o == null) {

             for (Node<E> x = last; x != null; x = x.prev) {

                 index--;

                 if (x.item == null)

                     return index;

             }

         } else {

             for (Node<E> x = last; x != null; x = x.prev) {

                 index--;

                 if (o.equals(x.item))

                     return index;

             }

         }

         return -1;

     }

     // Queue operations.

     /**

      * Retrieves, but does not remove, the head (first element) of this list.

      *

      * @return the head of this list, or {@code null} if this list is empty

      * @since 1.5

      */

     public E peek() {

         final Node<E> f = first;

         return (f == null) ? null : f.item;

     }

     /**

      * Retrieves, but does not remove, the head (first element) of this list.

      *

      * @return the head of this list

      * @throws NoSuchElementException if this list is empty

      * @since 1.5

      */

     public E element() {

         return getFirst();

     }

     /**

      * Retrieves and removes the head (first element) of this list.

      *

      * @return the head of this list, or {@code null} if this list is empty

      * @since 1.5

      */

     public E poll() {

         final Node<E> f = first;

         return (f == null) ? null : unlinkFirst(f);

     }

     /**

      * Retrieves and removes the head (first element) of this list.

      *

      * @return the head of this list

      * @throws NoSuchElementException if this list is empty

      * @since 1.5

      */

     public E remove() {

         return removeFirst();

     }

     /**

      * Adds the specified element as the tail (last element) of this list.

      *

      * @param e the element to add

      * @return {@code true} (as specified by {@link Queue#offer})

      * @since 1.5

      */

     public boolean offer(E e) {

         return add(e);

     }

     // Deque operations

     /**

      * Inserts the specified element at the front of this list.

      *

      * @param e the element to insert

      * @return {@code true} (as specified by {@link Deque#offerFirst})

      * @since 1.6

      */

     public boolean offerFirst(E e) {

         addFirst(e);

         return true;

     }

     /**

      * Inserts the specified element at the end of this list.

      *

      * @param e the element to insert

      * @return {@code true} (as specified by {@link Deque#offerLast})

      * @since 1.6

      */

     public boolean offerLast(E e) {

         addLast(e);

         return true;

     }

     /**

      * Retrieves, but does not remove, the first element of this list,

      * or returns {@code null} if this list is empty.

      *

      * @return the first element of this list, or {@code null}

      *         if this list is empty

      * @since 1.6

      */

     public E peekFirst() {

         final Node<E> f = first;

         return (f == null) ? null : f.item;

      }

     /**

      * Retrieves, but does not remove, the last element of this list,

      * or returns {@code null} if this list is empty.

      *

      * @return the last element of this list, or {@code null}

      *         if this list is empty

      * @since 1.6

      */

     public E peekLast() {

         final Node<E> l = last;

         return (l == null) ? null : l.item;

     }

     /**

      * Retrieves and removes the first element of this list,

      * or returns {@code null} if this list is empty.

      *

      * @return the first element of this list, or {@code null} if

      *     this list is empty

      * @since 1.6

      */

     public E pollFirst() {

         final Node<E> f = first;

         return (f == null) ? null : unlinkFirst(f);

     }

     /**

      * Retrieves and removes the last element of this list,

      * or returns {@code null} if this list is empty.

      *

      * @return the last element of this list, or {@code null} if

      *     this list is empty

      * @since 1.6

      */

     public E pollLast() {

         final Node<E> l = last;

         return (l == null) ? null : unlinkLast(l);

     }

     /**

      * Pushes an element onto the stack represented by this list.  In other

      * words, inserts the element at the front of this list.

      *

      * <p>This method is equivalent to {@link #addFirst}.

      *

      * @param e the element to push

      * @since 1.6

      */

     public void push(E e) {

         addFirst(e);

     }

     /**

      * Pops an element from the stack represented by this list.  In other

      * words, removes and returns the first element of this list.

      *

      * <p>This method is equivalent to {@link #removeFirst()}.

      *

      * @return the element at the front of this list (which is the top

      *         of the stack represented by this list)

      * @throws NoSuchElementException if this list is empty

      * @since 1.6

      */

     public E pop() {

         return removeFirst();

     }

     /**

      * Removes the first occurrence of the specified element in this

      * list (when traversing the list from head to tail).  If the list

      * does not contain the element, it is unchanged.

      *

      * @param o element to be removed from this list, if present

      * @return {@code true} if the list contained the specified element

      * @since 1.6

      */

     public boolean removeFirstOccurrence(Object o) {

         return remove(o);

     }

     /**

      * Removes the last occurrence of the specified element in this

      * list (when traversing the list from head to tail).  If the list

      * does not contain the element, it is unchanged.

      *

      * @param o element to be removed from this list, if present

      * @return {@code true} if the list contained the specified element

      * @since 1.6

      */

     public boolean removeLastOccurrence(Object o) {

         if (o == null) {

             for (Node<E> x = last; x != null; x = x.prev) {

                 if (x.item == null) {

                     unlink(x);

                     return true;

                 }

             }

         } else {

             for (Node<E> x = last; x != null; x = x.prev) {

                 if (o.equals(x.item)) {

                     unlink(x);

                     return true;

                 }

             }

         }

         return false;

     }

     /**

      * Returns a list-iterator of the elements in this list (in proper

      * sequence), starting at the specified position in the list.

      * Obeys the general contract of {@code List.listIterator(int)}.<p>

      *

      * The list-iterator is <i>fail-fast</i>: if the list is structurally

      * modified at any time after the Iterator is created, in any way except

      * through the list-iterator's own {@code remove} or {@code add}

      * methods, the list-iterator will throw a

      * {@code ConcurrentModificationException}.  Thus, in the face of

      * concurrent modification, the iterator fails quickly and cleanly, rather

      * than risking arbitrary, non-deterministic behavior at an undetermined

      * time in the future.

      *

      * @param index index of the first element to be returned from the

      *              list-iterator (by a call to {@code next})

      * @return a ListIterator of the elements in this list (in proper

      *         sequence), starting at the specified position in the list

      * @throws IndexOutOfBoundsException {@inheritDoc}

      * @see List#listIterator(int)

      */

     public ListIterator<E> listIterator(int index) {

         checkPositionIndex(index);

         return new ListItr(index);

     }

     private class ListItr implements ListIterator<E> {

         private Node<E> lastReturned = null;

         private Node<E> next;

         private int nextIndex;

         private int expectedModCount = modCount;

         ListItr(int index) {

             // assert isPositionIndex(index);

             next = (index == size) ? null : node(index);

             nextIndex = index;

         }

         public boolean hasNext() {

             return nextIndex < size;

         }

         public E next() {

             checkForComodification();

             if (!hasNext())

                 throw new NoSuchElementException();

             lastReturned = next;

             next = next.next;

             nextIndex++;

             return lastReturned.item;

         }

         public boolean hasPrevious() {

             return nextIndex > 0;

         }

         public E previous() {

             checkForComodification();

             if (!hasPrevious())

                 throw new NoSuchElementException();

             lastReturned = next = (next == null) ? last : next.prev;

             nextIndex--;

             return lastReturned.item;

         }

         public int nextIndex() {

             return nextIndex;

         }

         public int previousIndex() {

             return nextIndex - 1;

         }

         public void remove() {

             checkForComodification();

             if (lastReturned == null)

                 throw new IllegalStateException();

             Node<E> lastNext = lastReturned.next;

             unlink(lastReturned);

             if (next == lastReturned)

                 next = lastNext;

             else

                 nextIndex--;

             lastReturned = null;

             expectedModCount++;

         }

         public void set(E e) {

             if (lastReturned == null)

                 throw new IllegalStateException();

             checkForComodification();

             lastReturned.item = e;

         }

         public void add(E e) {

             checkForComodification();

             lastReturned = null;

             if (next == null)

                 linkLast(e);

             else

                 linkBefore(e, next);

             nextIndex++;

             expectedModCount++;

         }

         final void checkForComodification() {

             if (modCount != expectedModCount)

                 throw new ConcurrentModificationException();

         }

     }

     private static class Node<E> {

         E item;

         Node<E> next;

         Node<E> prev;

         Node(Node<E> prev, E element, Node<E> next) {

             this.item = element;

             this.next = next;

             this.prev = prev;

         }

     }

     /**

      * @since 1.6

      */

     public Iterator<E> descendingIterator() {

         return new DescendingIterator();

     }

     /**

      * Adapter to provide descending iterators via ListItr.previous

      */

     private class DescendingIterator implements Iterator<E> {

         private final ListItr itr = new ListItr(size());

         public boolean hasNext() {

             return itr.hasPrevious();

         }

         public E next() {

             return itr.previous();

         }

         public void remove() {

             itr.remove();

         }

     }

     @SuppressWarnings("unchecked")

     private LinkedList<E> superClone() {

         try {

             return (LinkedList<E>) super.clone();

         } catch (CloneNotSupportedException e) {

             throw new InternalError();

         }

     }

     /**

      * Returns a shallow copy of this {@code LinkedList}. (The elements

      * themselves are not cloned.)

      *

      * @return a shallow copy of this {@code LinkedList} instance

      */

     public Object clone() {

         LinkedList<E> clone = superClone();

         // Put clone into "virgin" state

         clone.first = clone.last = null;

         clone.size = 0;

         clone.modCount = 0;

         // Initialize clone with our elements

         for (Node<E> x = first; x != null; x = x.next)

             clone.add(x.item);

         return clone;

     }

     /**

      * Returns an array containing all of the elements in this list

      * in proper sequence (from first to last element).

      *

      * <p>The returned array will be "safe" in that no references to it are

      * maintained by this list.  (In other words, this method must allocate

      * a new array).  The caller is thus free to modify the returned array.

      *

      * <p>This method acts as bridge between array-based and collection-based

      * APIs.

      *

      * @return an array containing all of the elements in this list

      *         in proper sequence

      */

     public Object[] toArray() {

         Object[] result = new Object[size];

         int i = 0;

         for (Node<E> x = first; x != null; x = x.next)

             result[i++] = x.item;

         return result;

     }

     /**

      * Returns an array containing all of the elements in this list in

      * proper sequence (from first to last element); the runtime type of

      * the returned array is that of the specified array.  If the list fits

      * in the specified array, it is returned therein.  Otherwise, a new

      * array is allocated with the runtime type of the specified array and

      * the size of this list.

      *

      * <p>If the list fits in the specified array with room to spare (i.e.,

      * the array has more elements than the list), the element in the array

      * immediately following the end of the list is set to {@code null}.

      * (This is useful in determining the length of the list <i>only</i> if

      * the caller knows that the list does not contain any null elements.)

      *

      * <p>Like the {@link #toArray()} method, this method acts as bridge between

      * array-based and collection-based APIs.  Further, this method allows

      * precise control over the runtime type of the output array, and may,

      * under certain circumstances, be used to save allocation costs.

      *

      * <p>Suppose {@code x} is a list known to contain only strings.

      * The following code can be used to dump the list into a newly

      * allocated array of {@code String}:

      *

      * <pre>

      *     String[] y = x.toArray(new String[0]);</pre>

      *

      * Note that {@code toArray(new Object[0])} is identical in function to

      * {@code toArray()}.

      *

      * @param a the array into which the elements of the list are to

      *          be stored, if it is big enough; otherwise, a new array of the

      *          same runtime type is allocated for this purpose.

      * @return an array containing the elements of the list

      * @throws ArrayStoreException if the runtime type of the specified array

      *         is not a supertype of the runtime type of every element in

      *         this list

      * @throws NullPointerException if the specified array is null

      */

     @SuppressWarnings("unchecked")

     public <T> T[] toArray(T[] a) {

         if (a.length < size)

             a = (T[])java.lang.reflect.Array.newInstance(

                                 a.getClass().getComponentType(), size);

         int i = 0;

         Object[] result = a;

         for (Node<E> x = first; x != null; x = x.next)

             result[i++] = x.item;

         if (a.length > size)

             a[size] = null;

         return a;

     }

     private static final long serialVersionUID = 876323262645176354L;

 }