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package java.util; |
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/* |
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* Todo |
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* |
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* 1) Make it serializable. |
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*/ |
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/** |
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* An unbounded priority queue based on a priority heap. This queue orders |
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* elements according to the order specified at creation time. This order is |
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* specified as for {@link TreeSet} and {@link TreeMap}: Elements are ordered |
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* either according to their <i>natural order</i> (see {@link Comparable}), or |
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* according to a {@link Comparator}, depending on which constructor is used. |
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* The {@link #peek}, {@link #poll}, and {@link #remove} methods return the |
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* minimal element with respect to the specified ordering. If multiple |
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* these elements are tied for least value, no guarantees are made as to |
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* which of elements is returned. |
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* |
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* <p>Each priority queue has a <i>capacity</i>. The capacity is the size of |
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* the array used to store the elements on the queue. It is always at least |
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* as large as the queue size. As elements are added to a priority list, |
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* its capacity grows automatically. The details of the growth policy are not |
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* specified. |
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* |
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*<p>Implementation note: this implementation provides O(log(n)) time for |
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* the <tt>offer</tt>, <tt>poll</tt>, <tt>remove()</tt> and <tt>add</tt> |
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* methods; linear time for the <tt>remove(Object)</tt> and |
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* <tt>contains</tt> methods; and constant time for the <tt>peek</tt>, |
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* <tt>element</tt>, and <tt>size</tt> methods. |
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* |
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* <p>This class is a member of the |
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* <a href="{@docRoot}/../guide/collections/index.html"> |
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* Java Collections Framework</a>. |
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*/ |
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public class PriorityQueue<E> extends AbstractQueue<E> |
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implements Queue<E> |
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{ |
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private static final int DEFAULT_INITIAL_CAPACITY = 11; |
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/** |
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* Priority queue represented as a balanced binary heap: the two children |
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* of queue[n] are queue[2*n] and queue[2*n + 1]. The priority queue is |
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* ordered by comparator, or by the elements' natural ordering, if |
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* comparator is null: For each node n in the heap, and each descendant |
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* of n, d, n <= d. |
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* |
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* The element with the lowest value is in queue[1] (assuming the queue is |
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* nonempty). A one-based array is used in preference to the traditional |
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* zero-based array to simplify parent and child calculations. |
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* |
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* queue.length must be >= 2, even if size == 0. |
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*/ |
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private E[] queue; |
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/** |
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* The number of elements in the priority queue. |
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*/ |
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private int size = 0; |
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/** |
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* The comparator, or null if priority queue uses elements' |
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* natural ordering. |
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*/ |
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private final Comparator<E> comparator; |
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/** |
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* The number of times this priority queue has been |
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* <i>structurally modified</i>. See AbstractList for gory details. |
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*/ |
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private int modCount = 0; |
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/** |
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* Create a new priority queue with the default initial capacity (11) |
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* that orders its elements according to their natural ordering. |
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*/ |
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public PriorityQueue() { |
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this(DEFAULT_INITIAL_CAPACITY); |
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} |
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|
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/** |
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* Create a new priority queue with the specified initial capacity |
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* that orders its elements according to their natural ordering. |
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* |
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* @param initialCapacity the initial capacity for this priority queue. |
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*/ |
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public PriorityQueue(int initialCapacity) { |
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this(initialCapacity, null); |
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} |
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/** |
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* Create a new priority queue with the specified initial capacity (11) |
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* that orders its elements according to the specified comparator. |
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* |
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* @param initialCapacity the initial capacity for this priority queue. |
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* @param comparator the comparator used to order this priority queue. |
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*/ |
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public PriorityQueue(int initialCapacity, Comparator<E> comparator) { |
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if (initialCapacity < 1) |
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initialCapacity = 1; |
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queue = new E[initialCapacity + 1]; |
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this.comparator = comparator; |
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} |
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/** |
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* Create a new priority queue containing the elements in the specified |
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* collection. The priority queue has an initial capacity of 110% of the |
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* size of the specified collection. If the specified collection |
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* implements the {@link Sorted} interface, the priority queue will be |
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* sorted according to the same comparator, or according to its elements' |
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* natural order if the collection is sorted according to its elements' |
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* natural order. If the specified collection does not implement the |
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* <tt>Sorted</tt> interface, the priority queue is ordered according to |
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* its elements' natural order. |
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* |
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* @param initialElements the collection whose elements are to be placed |
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* into this priority queue. |
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* @throws ClassCastException if elements of the specified collection |
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* cannot be compared to one another according to the priority |
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* queue's ordering. |
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* @throws NullPointerException if the specified collection or an |
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* element of the specified collection is <tt>null</tt>. |
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*/ |
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public PriorityQueue(Collection<E> initialElements) { |
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int sz = initialElements.size(); |
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int initialCapacity = (int)Math.min((sz * 110L) / 100, |
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Integer.MAX_VALUE - 1); |
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if (initialCapacity < 1) |
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initialCapacity = 1; |
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queue = new E[initialCapacity + 1]; |
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/* Commented out to compile with generics compiler |
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if (initialElements instanceof Sorted) { |
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comparator = ((Sorted)initialElements).comparator(); |
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for (Iterator<E> i = initialElements.iterator(); i.hasNext(); ) |
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queue[++size] = i.next(); |
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} else { |
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*/ |
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{ |
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comparator = null; |
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for (Iterator<E> i = initialElements.iterator(); i.hasNext(); ) |
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add(i.next()); |
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} |
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1.1 |
} |
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1.2 |
// Queue Methods |
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/** |
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* Remove and return the minimal element from this priority queue if |
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* it contains one or more elements, otherwise <tt>null</tt>. The term |
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* <i>minimal</i> is defined according to this priority queue's order. |
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* |
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* @return the minimal element from this priority queue if it contains |
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* one or more elements, otherwise <tt>null</tt>. |
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*/ |
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public E poll() { |
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if (size == 0) |
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return null; |
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return remove(1); |
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1.1 |
} |
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1.2 |
|
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/** |
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* Return, but do not remove, the minimal element from the priority queue, |
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* or <tt>null</tt> if the queue is empty. The term <i>minimal</i> is |
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* defined according to this priority queue's order. This method returns |
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* the same object reference that would be returned by by the |
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* <tt>poll</tt> method. The two methods differ in that this method |
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* does not remove the element from the priority queue. |
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* |
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* @return the minimal element from this priority queue if it contains |
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* one or more elements, otherwise <tt>null</tt>. |
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*/ |
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public E peek() { |
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return queue[1]; |
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1.1 |
} |
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1.2 |
// Collection Methods |
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/** |
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* Removes a single instance of the specified element from this priority |
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* queue, if it is present. Returns true if this collection contained the |
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* specified element (or equivalently, if this collection changed as a |
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* result of the call). |
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* |
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* @param o element to be removed from this collection, if present. |
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* @return <tt>true</tt> if this collection changed as a result of the |
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* call |
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* @throws ClassCastException if the specified element cannot be compared |
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* with elements currently in the priority queue according |
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* to the priority queue's ordering. |
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* @throws NullPointerException if the specified element is null. |
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*/ |
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public boolean remove(Object element) { |
195 |
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if (element == null) |
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throw new NullPointerException(); |
197 |
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if (comparator == null) { |
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for (int i = 1; i <= size; i++) { |
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if (((Comparable)queue[i]).compareTo(element) == 0) { |
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remove(i); |
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return true; |
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} |
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} |
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} else { |
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for (int i = 1; i <= size; i++) { |
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if (comparator.compare(queue[i], (E) element) == 0) { |
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remove(i); |
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return true; |
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} |
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} |
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} |
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1.1 |
return false; |
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} |
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1.2 |
|
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/** |
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* Returns an iterator over the elements in this priority queue. The |
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* first element returned by this iterator is the same element that |
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* would be returned by a call to <tt>peek</tt>. |
220 |
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* |
221 |
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* @return an <tt>Iterator</tt> over the elements in this priority queue. |
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*/ |
223 |
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public Iterator<E> iterator() { |
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return new Itr(); |
225 |
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} |
226 |
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227 |
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private class Itr implements Iterator<E> { |
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/** |
229 |
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* Index (into queue array) of element to be returned by |
230 |
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* subsequent call to next. |
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*/ |
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int cursor = 1; |
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/** |
235 |
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* Index of element returned by most recent call to next or |
236 |
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* previous. Reset to 0 if this element is deleted by a call |
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* to remove. |
238 |
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*/ |
239 |
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int lastRet = 0; |
240 |
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/** |
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* The modCount value that the iterator believes that the backing |
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* List should have. If this expectation is violated, the iterator |
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* has detected concurrent modification. |
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*/ |
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int expectedModCount = modCount; |
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public boolean hasNext() { |
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return cursor <= size; |
250 |
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} |
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public E next() { |
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checkForComodification(); |
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if (cursor > size) |
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throw new NoSuchElementException(); |
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E result = queue[cursor]; |
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lastRet = cursor++; |
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return result; |
259 |
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} |
260 |
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261 |
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public void remove() { |
262 |
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if (lastRet == 0) |
263 |
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throw new IllegalStateException(); |
264 |
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checkForComodification(); |
265 |
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266 |
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PriorityQueue.this.remove(lastRet); |
267 |
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if (lastRet < cursor) |
268 |
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cursor--; |
269 |
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lastRet = 0; |
270 |
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expectedModCount = modCount; |
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} |
272 |
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273 |
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final void checkForComodification() { |
274 |
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if (modCount != expectedModCount) |
275 |
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throw new ConcurrentModificationException(); |
276 |
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} |
277 |
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} |
278 |
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279 |
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/** |
280 |
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* Returns the number of elements in this priority queue. |
281 |
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* |
282 |
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* @return the number of elements in this priority queue. |
283 |
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*/ |
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1.1 |
public int size() { |
285 |
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1.2 |
return size; |
286 |
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1.1 |
} |
287 |
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1.2 |
|
288 |
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/** |
289 |
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* Add the specified element to this priority queue. |
290 |
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* |
291 |
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* @param element the element to add. |
292 |
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* @return true |
293 |
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* @throws ClassCastException if the specified element cannot be compared |
294 |
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* with elements currently in the priority queue according |
295 |
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* to the priority queue's ordering. |
296 |
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* @throws NullPointerException if the specified element is null. |
297 |
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*/ |
298 |
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public boolean offer(E element) { |
299 |
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if (element == null) |
300 |
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throw new NullPointerException(); |
301 |
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modCount++; |
302 |
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303 |
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// Grow backing store if necessary |
304 |
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if (++size == queue.length) { |
305 |
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E[] newQueue = new E[2 * queue.length]; |
306 |
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System.arraycopy(queue, 0, newQueue, 0, size); |
307 |
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queue = newQueue; |
308 |
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} |
309 |
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310 |
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queue[size] = element; |
311 |
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fixUp(size); |
312 |
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return true; |
313 |
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1.1 |
} |
314 |
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315 |
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1.2 |
/** |
316 |
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* Remove all elements from the priority queue. |
317 |
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*/ |
318 |
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public void clear() { |
319 |
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modCount++; |
320 |
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321 |
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// Null out element references to prevent memory leak |
322 |
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for (int i=1; i<=size; i++) |
323 |
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queue[i] = null; |
324 |
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325 |
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size = 0; |
326 |
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} |
327 |
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328 |
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/** |
329 |
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* Removes and returns the ith element from queue. Recall |
330 |
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* that queue is one-based, so 1 <= i <= size. |
331 |
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* |
332 |
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* XXX: Could further special-case i==size, but is it worth it? |
333 |
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* XXX: Could special-case i==0, but is it worth it? |
334 |
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*/ |
335 |
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private E remove(int i) { |
336 |
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assert i <= size; |
337 |
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modCount++; |
338 |
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339 |
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E result = queue[i]; |
340 |
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queue[i] = queue[size]; |
341 |
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queue[size--] = null; // Drop extra ref to prevent memory leak |
342 |
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if (i <= size) |
343 |
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fixDown(i); |
344 |
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return result; |
345 |
tim |
1.1 |
} |
346 |
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347 |
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1.2 |
/** |
348 |
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* Establishes the heap invariant (described above) assuming the heap |
349 |
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* satisfies the invariant except possibly for the leaf-node indexed by k |
350 |
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* (which may have a nextExecutionTime less than its parent's). |
351 |
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* |
352 |
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* This method functions by "promoting" queue[k] up the hierarchy |
353 |
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* (by swapping it with its parent) repeatedly until queue[k] |
354 |
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* is greater than or equal to its parent. |
355 |
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*/ |
356 |
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private void fixUp(int k) { |
357 |
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if (comparator == null) { |
358 |
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while (k > 1) { |
359 |
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int j = k >> 1; |
360 |
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if (((Comparable)queue[j]).compareTo(queue[k]) <= 0) |
361 |
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break; |
362 |
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E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp; |
363 |
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k = j; |
364 |
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} |
365 |
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} else { |
366 |
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while (k > 1) { |
367 |
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int j = k >> 1; |
368 |
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if (comparator.compare(queue[j], queue[k]) <= 0) |
369 |
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break; |
370 |
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E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp; |
371 |
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k = j; |
372 |
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} |
373 |
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} |
374 |
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} |
375 |
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376 |
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/** |
377 |
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* Establishes the heap invariant (described above) in the subtree |
378 |
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* rooted at k, which is assumed to satisfy the heap invariant except |
379 |
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* possibly for node k itself (which may be greater than its children). |
380 |
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* |
381 |
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* This method functions by "demoting" queue[k] down the hierarchy |
382 |
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* (by swapping it with its smaller child) repeatedly until queue[k] |
383 |
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* is less than or equal to its children. |
384 |
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*/ |
385 |
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private void fixDown(int k) { |
386 |
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int j; |
387 |
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if (comparator == null) { |
388 |
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while ((j = k << 1) <= size) { |
389 |
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if (j<size && ((Comparable)queue[j]).compareTo(queue[j+1]) > 0) |
390 |
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j++; // j indexes smallest kid |
391 |
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if (((Comparable)queue[k]).compareTo(queue[j]) <= 0) |
392 |
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break; |
393 |
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E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp; |
394 |
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k = j; |
395 |
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} |
396 |
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} else { |
397 |
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while ((j = k << 1) <= size) { |
398 |
|
|
if (j < size && comparator.compare(queue[j], queue[j+1]) > 0) |
399 |
|
|
j++; // j indexes smallest kid |
400 |
|
|
if (comparator.compare(queue[k], queue[j]) <= 0) |
401 |
|
|
break; |
402 |
|
|
E tmp = queue[j]; queue[j] = queue[k]; queue[k] = tmp; |
403 |
|
|
k = j; |
404 |
|
|
} |
405 |
|
|
} |
406 |
|
|
} |
407 |
|
|
|
408 |
|
|
/** |
409 |
|
|
* Returns the comparator associated with this priority queue, or |
410 |
|
|
* <tt>null</tt> if it uses its elements' natural ordering. |
411 |
|
|
* |
412 |
|
|
* @return the comparator associated with this priority queue, or |
413 |
|
|
* <tt>null</tt> if it uses its elements' natural ordering. |
414 |
|
|
*/ |
415 |
tim |
1.3 |
Comparator<E> comparator() { |
416 |
tim |
1.2 |
return comparator; |
417 |
|
|
} |
418 |
tim |
1.1 |
} |