TreeSet，基于TreeMap实现

TreeSet基于TreeMap实现，支持排序；

TreeSet是非线程安全的；

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public class TreeSet<E> extends AbstractSet<E>    implements NavigableSet<E>, Cloneable, java.io.Serializable{    /**     * The backing map.     */    private transient NavigableMap<E,Object> m;    // Dummy value to associate with an Object in the backing Map    private static final Object PRESENT = new Object();    /**     * Constructs a set backed by the specified navigable map.     */    TreeSet(NavigableMap<E,Object> m) {        this.m = m;    }    /**     * Constructs a new, empty tree set, sorted according to the     * natural ordering of its elements.  All elements inserted into     * the set must implement the {@link Comparable} interface.     * Furthermore, all such elements must be <i>mutually     * comparable</i>: {@code e1.compareTo(e2)} must not throw a     * {@code ClassCastException} for any elements {@code e1} and     * {@code e2} in the set.  If the user attempts to add an element     * to the set that violates this constraint (for example, the user     * attempts to add a string element to a set whose elements are     * integers), the {@code add} call will throw a     * {@code ClassCastException}.     */    public TreeSet() {this(new TreeMap<E,Object>());    }

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/**     * Adds the specified element to this set if it is not already present.     * More formally, adds the specified element <tt>e</tt> to this set if     * this set contains no element <tt>e2</tt> such that     * <tt>(e==null ? e2==null : e.equals(e2))</tt>.     * If this set already contains the element, the call leaves the set     * unchanged and returns <tt>false</tt>.     */

public boolean add(E e) {return m.put(e, PRESENT)==null;    }

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/**     * Removes the specified element from this set if it is present.     * More formally, removes an element <tt>e</tt> such that     * <tt>(o==null ? e==null : o.equals(e))</tt>,     * if this set contains such an element.  Returns <tt>true</tt> if     * this set contained the element (or equivalently, if this set     * changed as a result of the call).  (This set will not contain the     * element once the call returns.)     */    public boolean remove(Object o) {return map.remove(o)==PRESENT;    }