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	/*
	* 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.
	*/

	/*
	* This file is available under and governed by the GNU General Public
	* License version 2 only, as published by the Free Software Foundation.
	* However, the following notice accompanied the original version of this
	* file:
	*
	* Written by Doug Lea and Josh Bloch with assistance from members of
	* JCP JSR-166 Expert Group and released to the public domain, as explained
	* at http://creativecommons.org/publicdomain/zero/1.0/
	*/

	package java.util;

	/**
	* A linear collection that supports element insertion and removal at
	* both ends. The name <i>deque</i> is short for "double ended queue"
	* and is usually pronounced "deck". Most {@code Deque}
	* implementations place no fixed limits on the number of elements
	* they may contain, but this interface supports capacity-restricted
	* deques as well as those with no fixed size limit.
	*
	* <p>This interface defines methods to access the elements at both
	* ends of the deque. Methods are provided to insert, remove, and
	* examine the element. Each of these methods exists in two forms:
	* one throws an exception if the operation fails, the other returns a
	* special value (either {@code null} or {@code false}, depending on
	* the operation). The latter form of the insert operation is
	* designed specifically for use with capacity-restricted
	* {@code Deque} implementations; in most implementations, insert
	* operations cannot fail.
	*
	* <p>The twelve methods described above are summarized in the
	* following table:
	*
	* <table class="striped">
	* <caption>Summary of Deque methods</caption>
	* <thead>
	* <tr>
	* <td rowspan="2"></td>
	* <th scope="col" colspan="2"> First Element (Head)</th>
	* <th scope="col" colspan="2"> Last Element (Tail)</th>
	* </tr>
	* <tr>
	* <th scope="col" style="font-weight:normal; font-style:italic">Throws exception</th>
	* <th scope="col" style="font-weight:normal; font-style:italic">Special value</th>
	* <th scope="col" style="font-weight:normal; font-style:italic">Throws exception</th>
	* <th scope="col" style="font-weight:normal; font-style:italic">Special value</th>
	* </tr>
	* </thead>
	* <tbody>
	* <tr>
	* <th scope="row">Insert</th>
	* <td>{@link #addFirst(Object) addFirst(e)}</td>
	* <td>{@link #offerFirst(Object) offerFirst(e)}</td>
	* <td>{@link #addLast(Object) addLast(e)}</td>
	* <td>{@link #offerLast(Object) offerLast(e)}</td>
	* </tr>
	* <tr>
	* <th scope="row">Remove</th>
	* <td>{@link #removeFirst() removeFirst()}</td>
	* <td>{@link #pollFirst() pollFirst()}</td>
	* <td>{@link #removeLast() removeLast()}</td>
	* <td>{@link #pollLast() pollLast()}</td>
	* </tr>
	* <tr>
	* <th scope="row">Examine</th>
	* <td>{@link #getFirst() getFirst()}</td>
	* <td>{@link #peekFirst() peekFirst()}</td>
	* <td>{@link #getLast() getLast()}</td>
	* <td>{@link #peekLast() peekLast()}</td>
	* </tr>
	* </tbody>
	* </table>
	*
	* <p>This interface extends the {@link Queue} interface. When a deque is
	* used as a queue, FIFO (First-In-First-Out) behavior results. Elements are
	* added at the end of the deque and removed from the beginning. The methods
	* inherited from the {@code Queue} interface are precisely equivalent to
	* {@code Deque} methods as indicated in the following table:
	*
	* <table class="striped">
	* <caption>Comparison of Queue and Deque methods</caption>
	* <thead>
	* <tr>
	* <th scope="col"> {@code Queue} Method</th>
	* <th scope="col"> Equivalent {@code Deque} Method</th>
	* </tr>
	* </thead>
	* <tbody>
	* <tr>
	* <th scope="row">{@link #add(Object) add(e)}</th>
	* <td>{@link #addLast(Object) addLast(e)}</td>
	* </tr>
	* <tr>
	* <th scope="row">{@link #offer(Object) offer(e)}</th>
	* <td>{@link #offerLast(Object) offerLast(e)}</td>
	* </tr>
	* <tr>
	* <th scope="row">{@link #remove() remove()}</th>
	* <td>{@link #removeFirst() removeFirst()}</td>
	* </tr>
	* <tr>
	* <th scope="row">{@link #poll() poll()}</th>
	* <td>{@link #pollFirst() pollFirst()}</td>
	* </tr>
	* <tr>
	* <th scope="row">{@link #element() element()}</th>
	* <td>{@link #getFirst() getFirst()}</td>
	* </tr>
	* <tr>
	* <th scope="row">{@link #peek() peek()}</th>
	* <td>{@link #peekFirst() peekFirst()}</td>
	* </tr>
	* </tbody>
	* </table>
	*
	* <p>Deques can also be used as LIFO (Last-In-First-Out) stacks. This
	* interface should be used in preference to the legacy {@link Stack} class.
	* When a deque is used as a stack, elements are pushed and popped from the
	* beginning of the deque. Stack methods are equivalent to {@code Deque}
	* methods as indicated in the table below:
	*
	* <table class="striped">
	* <caption>Comparison of Stack and Deque methods</caption>
	* <thead>
	* <tr>
	* <th scope="col"> Stack Method</th>
	* <th scope="col"> Equivalent {@code Deque} Method</th>
	* </tr>
	* </thead>
	* <tbody>
	* <tr>
	* <th scope="row">{@link #push(Object) push(e)}</th>
	* <td>{@link #addFirst(Object) addFirst(e)}</td>
	* </tr>
	* <tr>
	* <th scope="row">{@link #pop() pop()}</th>
	* <td>{@link #removeFirst() removeFirst()}</td>
	* </tr>
	* <tr>
	* <th scope="row">{@link #peek() peek()}</th>
	* <td>{@link #getFirst() getFirst()}</td>
	* </tr>
	* </tbody>
	* </table>
	*
	* <p>Note that the {@link #peek peek} method works equally well when
	* a deque is used as a queue or a stack; in either case, elements are
	* drawn from the beginning of the deque.
	*
	* <p>This interface provides two methods to remove interior
	* elements, {@link #removeFirstOccurrence removeFirstOccurrence} and
	* {@link #removeLastOccurrence removeLastOccurrence}.
	*
	* <p>Unlike the {@link List} interface, this interface does not
	* provide support for indexed access to elements.
	*
	* <p>While {@code Deque} implementations are not strictly required
	* to prohibit the insertion of null elements, they are strongly
	* encouraged to do so. Users of any {@code Deque} implementations
	* that do allow null elements are strongly encouraged <i>not</i> to
	* take advantage of the ability to insert nulls. This is so because
	* {@code null} is used as a special return value by various methods
	* to indicate that the deque is empty.
	*
	* <p>{@code Deque} implementations generally do not define
	* element-based versions of the {@code equals} and {@code hashCode}
	* methods, but instead inherit the identity-based versions from class
	* {@code Object}.
	*
	* <p>This interface is a member of the
	* <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
	* Java Collections Framework</a>.
	*
	* @author Doug Lea
	* @author Josh Bloch
	* @since 1.6
	* @param <E> the type of elements held in this deque
	*/
	public interface Deque<E> extends Queue<E> {
	/**
	* Inserts the specified element at the front of this deque if it is
	* possible to do so immediately without violating capacity restrictions,
	* throwing an {@code IllegalStateException} if no space is currently
	* available. When using a capacity-restricted deque, it is generally
	* preferable to use method {@link #offerFirst}.
	*
	* @param e the element to add
	* @throws IllegalStateException if the element cannot be added at this
	* time due to capacity restrictions
	* @throws ClassCastException if the class of the specified element
	* prevents it from being added to this deque
	* @throws NullPointerException if the specified element is null and this
	* deque does not permit null elements
	* @throws IllegalArgumentException if some property of the specified
	* element prevents it from being added to this deque
	*/
	void addFirst(E e);

	/**
	* Inserts the specified element at the end of this deque if it is
	* possible to do so immediately without violating capacity restrictions,
	* throwing an {@code IllegalStateException} if no space is currently
	* available. When using a capacity-restricted deque, it is generally
	* preferable to use method {@link #offerLast}.
	*
	* <p>This method is equivalent to {@link #add}.
	*
	* @param e the element to add
	* @throws IllegalStateException if the element cannot be added at this
	* time due to capacity restrictions
	* @throws ClassCastException if the class of the specified element
	* prevents it from being added to this deque
	* @throws NullPointerException if the specified element is null and this
	* deque does not permit null elements
	* @throws IllegalArgumentException if some property of the specified
	* element prevents it from being added to this deque
	*/
	void addLast(E e);

	/**
	* Inserts the specified element at the front of this deque unless it would
	* violate capacity restrictions. When using a capacity-restricted deque,
	* this method is generally preferable to the {@link #addFirst} method,
	* which can fail to insert an element only by throwing an exception.
	*
	* @param e the element to add
	* @return {@code true} if the element was added to this deque, else
	* {@code false}
	* @throws ClassCastException if the class of the specified element
	* prevents it from being added to this deque
	* @throws NullPointerException if the specified element is null and this
	* deque does not permit null elements
	* @throws IllegalArgumentException if some property of the specified
	* element prevents it from being added to this deque
	*/
	boolean offerFirst(E e);

	/**
	* Inserts the specified element at the end of this deque unless it would
	* violate capacity restrictions. When using a capacity-restricted deque,
	* this method is generally preferable to the {@link #addLast} method,
	* which can fail to insert an element only by throwing an exception.
	*
	* @param e the element to add
	* @return {@code true} if the element was added to this deque, else
	* {@code false}
	* @throws ClassCastException if the class of the specified element
	* prevents it from being added to this deque
	* @throws NullPointerException if the specified element is null and this
	* deque does not permit null elements
	* @throws IllegalArgumentException if some property of the specified
	* element prevents it from being added to this deque
	*/
	boolean offerLast(E e);

	/**
	* Retrieves and removes the first element of this deque. This method
	* differs from {@link #pollFirst pollFirst} only in that it throws an
	* exception if this deque is empty.
	*
	* @return the head of this deque
	* @throws NoSuchElementException if this deque is empty
	*/
	E removeFirst();

	/**
	* Retrieves and removes the last element of this deque. This method
	* differs from {@link #pollLast pollLast} only in that it throws an
	* exception if this deque is empty.
	*
	* @return the tail of this deque
	* @throws NoSuchElementException if this deque is empty
	*/
	E removeLast();

	/**
	* Retrieves and removes the first element of this deque,
	* or returns {@code null} if this deque is empty.
	*
	* @return the head of this deque, or {@code null} if this deque is empty
	*/
	E pollFirst();

	/**
	* Retrieves and removes the last element of this deque,
	* or returns {@code null} if this deque is empty.
	*
	* @return the tail of this deque, or {@code null} if this deque is empty
	*/
	E pollLast();

	/**
	* Retrieves, but does not remove, the first element of this deque.
	*
	* This method differs from {@link #peekFirst peekFirst} only in that it
	* throws an exception if this deque is empty.
	*
	* @return the head of this deque
	* @throws NoSuchElementException if this deque is empty
	*/
	E getFirst();

	/**
	* Retrieves, but does not remove, the last element of this deque.
	* This method differs from {@link #peekLast peekLast} only in that it
	* throws an exception if this deque is empty.
	*
	* @return the tail of this deque
	* @throws NoSuchElementException if this deque is empty
	*/
	E getLast();

	/**
	* Retrieves, but does not remove, the first element of this deque,
	* or returns {@code null} if this deque is empty.
	*
	* @return the head of this deque, or {@code null} if this deque is empty
	*/
	E peekFirst();

	/**
	* Retrieves, but does not remove, the last element of this deque,
	* or returns {@code null} if this deque is empty.
	*
	* @return the tail of this deque, or {@code null} if this deque is empty
	*/
	E peekLast();

	/**
	* Removes the first occurrence of the specified element from this deque.
	* If the deque does not contain the element, it is unchanged.
	* More formally, removes the first element {@code e} such that
	* {@code Objects.equals(o, e)} (if such an element exists).
	* Returns {@code true} if this deque contained the specified element
	* (or equivalently, if this deque changed as a result of the call).
	*
	* @param o element to be removed from this deque, if present
	* @return {@code true} if an element was removed as a result of this call
	* @throws ClassCastException if the class of the specified element
	* is incompatible with this deque
	* (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
	* @throws NullPointerException if the specified element is null and this
	* deque does not permit null elements
	* (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
	*/
	boolean removeFirstOccurrence(Object o);

	/**
	* Removes the last occurrence of the specified element from this deque.
	* If the deque does not contain the element, it is unchanged.
	* More formally, removes the last element {@code e} such that
	* {@code Objects.equals(o, e)} (if such an element exists).
	* Returns {@code true} if this deque contained the specified element
	* (or equivalently, if this deque changed as a result of the call).
	*
	* @param o element to be removed from this deque, if present
	* @return {@code true} if an element was removed as a result of this call
	* @throws ClassCastException if the class of the specified element
	* is incompatible with this deque
	* (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
	* @throws NullPointerException if the specified element is null and this
	* deque does not permit null elements
	* (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
	*/
	boolean removeLastOccurrence(Object o);

	// * Queue methods *

	/**
	* Inserts the specified element into the queue represented by this deque
	* (in other words, at the tail of this deque) if it is possible to do so
	* immediately without violating capacity restrictions, returning
	* {@code true} upon success and throwing an
	* {@code IllegalStateException} if no space is currently available.
	* When using a capacity-restricted deque, it is generally preferable to
	* use {@link #offer(Object) offer}.
	*
	* <p>This method is equivalent to {@link #addLast}.
	*
	* @param e the element to add
	* @return {@code true} (as specified by {@link Collection#add})
	* @throws IllegalStateException if the element cannot be added at this
	* time due to capacity restrictions
	* @throws ClassCastException if the class of the specified element
	* prevents it from being added to this deque
	* @throws NullPointerException if the specified element is null and this
	* deque does not permit null elements
	* @throws IllegalArgumentException if some property of the specified
	* element prevents it from being added to this deque
	*/
	boolean add(E e);

	/**
	* Inserts the specified element into the queue represented by this deque
	* (in other words, at the tail of this deque) if it is possible to do so
	* immediately without violating capacity restrictions, returning
	* {@code true} upon success and {@code false} if no space is currently
	* available. When using a capacity-restricted deque, this method is
	* generally preferable to the {@link #add} method, which can fail to
	* insert an element only by throwing an exception.
	*
	* <p>This method is equivalent to {@link #offerLast}.
	*
	* @param e the element to add
	* @return {@code true} if the element was added to this deque, else
	* {@code false}
	* @throws ClassCastException if the class of the specified element
	* prevents it from being added to this deque
	* @throws NullPointerException if the specified element is null and this
	* deque does not permit null elements
	* @throws IllegalArgumentException if some property of the specified
	* element prevents it from being added to this deque
	*/
	boolean offer(E e);

	/**
	* Retrieves and removes the head of the queue represented by this deque
	* (in other words, the first element of this deque).
	* This method differs from {@link #poll() poll()} only in that it
	* throws an exception if this deque is empty.
	*
	* <p>This method is equivalent to {@link #removeFirst()}.
	*
	* @return the head of the queue represented by this deque
	* @throws NoSuchElementException if this deque is empty
	*/
	E remove();

	/**
	* Retrieves and removes the head of the queue represented by this deque
	* (in other words, the first element of this deque), or returns
	* {@code null} if this deque is empty.
	*
	* <p>This method is equivalent to {@link #pollFirst()}.
	*
	* @return the first element of this deque, or {@code null} if
	* this deque is empty
	*/
	E poll();

	/**
	* Retrieves, but does not remove, the head of the queue represented by
	* this deque (in other words, the first element of this deque).
	* This method differs from {@link #peek peek} only in that it throws an
	* exception if this deque is empty.
	*
	* <p>This method is equivalent to {@link #getFirst()}.
	*
	* @return the head of the queue represented by this deque
	* @throws NoSuchElementException if this deque is empty
	*/
	E element();

	/**
	* Retrieves, but does not remove, the head of the queue represented by
	* this deque (in other words, the first element of this deque), or
	* returns {@code null} if this deque is empty.
	*
	* <p>This method is equivalent to {@link #peekFirst()}.
	*
	* @return the head of the queue represented by this deque, or
	* {@code null} if this deque is empty
	*/
	E peek();

	/**
	* Adds all of the elements in the specified collection at the end
	* of this deque, as if by calling {@link #addLast} on each one,
	* in the order that they are returned by the collection's iterator.
	*
	* <p>When using a capacity-restricted deque, it is generally preferable
	* to call {@link #offer(Object) offer} separately on each element.
	*
	* <p>An exception encountered while trying to add an element may result
	* in only some of the elements having been successfully added when
	* the associated exception is thrown.
	*
	* @param c the elements to be inserted into this deque
	* @return {@code true} if this deque changed as a result of the call
	* @throws IllegalStateException if not all the elements can be added at
	* this time due to insertion restrictions
	* @throws ClassCastException if the class of an element of the specified
	* collection prevents it from being added to this deque
	* @throws NullPointerException if the specified collection contains a
	* null element and this deque does not permit null elements,
	* or if the specified collection is null
	* @throws IllegalArgumentException if some property of an element of the
	* specified collection prevents it from being added to this deque
	*/
	boolean addAll(Collection<? extends E> c);

	// * Stack methods *

	/**
	* Pushes an element onto the stack represented by this deque (in other
	* words, at the head of this deque) if it is possible to do so
	* immediately without violating capacity restrictions, throwing an
	* {@code IllegalStateException} if no space is currently available.
	*
	* <p>This method is equivalent to {@link #addFirst}.
	*
	* @param e the element to push
	* @throws IllegalStateException if the element cannot be added at this
	* time due to capacity restrictions
	* @throws ClassCastException if the class of the specified element
	* prevents it from being added to this deque
	* @throws NullPointerException if the specified element is null and this
	* deque does not permit null elements
	* @throws IllegalArgumentException if some property of the specified
	* element prevents it from being added to this deque
	*/
	void push(E e);

	/**
	* Pops an element from the stack represented by this deque. In other
	* words, removes and returns the first element of this deque.
	*
	* <p>This method is equivalent to {@link #removeFirst()}.
	*
	* @return the element at the front of this deque (which is the top
	* of the stack represented by this deque)
	* @throws NoSuchElementException if this deque is empty
	*/
	E pop();


	// * Collection methods *

	/**
	* Removes the first occurrence of the specified element from this deque.
	* If the deque does not contain the element, it is unchanged.
	* More formally, removes the first element {@code e} such that
	* {@code Objects.equals(o, e)} (if such an element exists).
	* Returns {@code true} if this deque contained the specified element
	* (or equivalently, if this deque changed as a result of the call).
	*
	* <p>This method is equivalent to {@link #removeFirstOccurrence(Object)}.
	*
	* @param o element to be removed from this deque, if present
	* @return {@code true} if an element was removed as a result of this call
	* @throws ClassCastException if the class of the specified element
	* is incompatible with this deque
	* (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
	* @throws NullPointerException if the specified element is null and this
	* deque does not permit null elements
	* (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
	*/
	boolean remove(Object o);

	/**
	* Returns {@code true} if this deque contains the specified element.
	* More formally, returns {@code true} if and only if this deque contains
	* at least one element {@code e} such that {@code Objects.equals(o, e)}.
	*
	* @param o element whose presence in this deque is to be tested
	* @return {@code true} if this deque contains the specified element
	* @throws ClassCastException if the class of the specified element
	* is incompatible with this deque
	* (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
	* @throws NullPointerException if the specified element is null and this
	* deque does not permit null elements
	* (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)
	*/
	boolean contains(Object o);

	/**
	* Returns the number of elements in this deque.
	*
	* @return the number of elements in this deque
	*/
	int size();

	/**
	* Returns an iterator over the elements in this deque in proper sequence.
	* The elements will be returned in order from first (head) to last (tail).
	*
	* @return an iterator over the elements in this deque in proper sequence
	*/
	Iterator<E> iterator();

	/**
	* Returns an iterator over the elements in this deque in reverse
	* sequential order. The elements will be returned in order from
	* last (tail) to first (head).
	*
	* @return an iterator over the elements in this deque in reverse
	* sequence
	*/
	Iterator<E> descendingIterator();

	}

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