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	/*
	* Copyright (c) 1997, 2019, 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
	* or visit www.oracle.com if you need additional information or have any
	* questions.
	*/

	package java.util;

	import java.util.function.Consumer;

	/**
	* 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}/java.base/java/util/package-summary.html#CollectionsFramework">
	* 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.
	*/
	transient Node<E> first;

	/**
	* Pointer to last node.
	*/
	transient Node<E> last;

	/*
	void dataStructureInvariants() {
	assert (size == 0)
	? (first == null && last == null)
	: (first.prev == null && last.next == null);
	}
	*/

	/**
	* 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
	* {@code Objects.equals(o, e)}.
	*
	* @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) >= 0;
	}

	/**
	* 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
	* {@code Objects.equals(o, get(i))}
	* (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
	* {@code Objects.equals(o, get(i))},
	* 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
	* {@code Objects.equals(o, get(i))},
	* 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;
	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++;
	}

	public void forEachRemaining(Consumer<? super E> action) {
	Objects.requireNonNull(action);
	while (modCount == expectedModCount && nextIndex < size) {
	action.accept(next.item);
	lastReturned = next;
	next = next.next;
	nextIndex++;
	}
	checkForComodification();
	}

	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(e);
	}
	}

	/**
	* 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;
	}

	@java.io.Serial
	private static final long serialVersionUID = 876323262645176354L;

	/**
	* Saves the state of this {@code LinkedList} instance to a stream
	* (that is, serializes it).
	*
	* @serialData The size of the list (the number of elements it
	* contains) is emitted (int), followed by all of its
	* elements (each an Object) in the proper order.
	*/
	@java.io.Serial
	private void writeObject(java.io.ObjectOutputStream s)
	throws java.io.IOException {
	// Write out any hidden serialization magic
	s.defaultWriteObject();

	// Write out size
	s.writeInt(size);

	// Write out all elements in the proper order.
	for (Node<E> x = first; x != null; x = x.next)
	s.writeObject(x.item);
	}

	/**
	* Reconstitutes this {@code LinkedList} instance from a stream
	* (that is, deserializes it).
	*/
	@SuppressWarnings("unchecked")
	@java.io.Serial
	private void readObject(java.io.ObjectInputStream s)
	throws java.io.IOException, ClassNotFoundException {
	// Read in any hidden serialization magic
	s.defaultReadObject();

	// Read in size
	int size = s.readInt();

	// Read in all elements in the proper order.
	for (int i = 0; i < size; i++)
	linkLast((E)s.readObject());
	}

	/**
	* Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>
	* and <em>fail-fast</em> {@link Spliterator} over the elements in this
	* list.
	*
	* <p>The {@code Spliterator} reports {@link Spliterator#SIZED} and
	* {@link Spliterator#ORDERED}. Overriding implementations should document
	* the reporting of additional characteristic values.
	*
	* @implNote
	* The {@code Spliterator} additionally reports {@link Spliterator#SUBSIZED}
	* and implements {@code trySplit} to permit limited parallelism..
	*
	* @return a {@code Spliterator} over the elements in this list
	* @since 1.8
	*/
	@Override
	public Spliterator<E> spliterator() {
	return new LLSpliterator<>(this, -1, 0);
	}

	/** A customized variant of Spliterators.IteratorSpliterator */
	static final class LLSpliterator<E> implements Spliterator<E> {
	static final int BATCH_UNIT = 1 << 10; // batch array size increment
	static final int MAX_BATCH = 1 << 25; // max batch array size;
	final LinkedList<E> list; // null OK unless traversed
	Node<E> current; // current node; null until initialized
	int est; // size estimate; -1 until first needed
	int expectedModCount; // initialized when est set
	int batch; // batch size for splits

	LLSpliterator(LinkedList<E> list, int est, int expectedModCount) {
	this.list = list;
	this.est = est;
	this.expectedModCount = expectedModCount;
	}

	final int getEst() {
	int s; // force initialization
	final LinkedList<E> lst;
	if ((s = est) < 0) {
	if ((lst = list) == null)
	s = est = 0;
	else {
	expectedModCount = lst.modCount;
	current = lst.first;
	s = est = lst.size;
	}
	}
	return s;
	}

	public long estimateSize() { return (long) getEst(); }

	public Spliterator<E> trySplit() {
	Node<E> p;
	int s = getEst();
	if (s > 1 && (p = current) != null) {
	int n = batch + BATCH_UNIT;
	if (n > s)
	n = s;
	if (n > MAX_BATCH)
	n = MAX_BATCH;
	Object[] a = new Object[n];
	int j = 0;
	do { a[j++] = p.item; } while ((p = p.next) != null && j < n);
	current = p;
	batch = j;
	est = s - j;
	return Spliterators.spliterator(a, 0, j, Spliterator.ORDERED);
	}
	return null;
	}

	public void forEachRemaining(Consumer<? super E> action) {
	Node<E> p; int n;
	if (action == null) throw new NullPointerException();
	if ((n = getEst()) > 0 && (p = current) != null) {
	current = null;
	est = 0;
	do {
	E e = p.item;
	p = p.next;
	action.accept(e);
	} while (p != null && --n > 0);
	}
	if (list.modCount != expectedModCount)
	throw new ConcurrentModificationException();
	}

	public boolean tryAdvance(Consumer<? super E> action) {
	Node<E> p;
	if (action == null) throw new NullPointerException();
	if (getEst() > 0 && (p = current) != null) {
	--est;
	E e = p.item;
	current = p.next;
	action.accept(e);
	if (list.modCount != expectedModCount)
	throw new ConcurrentModificationException();
	return true;
	}
	return false;
	}

	public int characteristics() {
	return Spliterator.ORDERED \| Spliterator.SIZED \| Spliterator.SUBSIZED;
	}
	}

	}

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