http://www.docjar.com/docs/api/java/util/technotes/guides/collections/changes4.html
http://www.docjar.com/docs/api/java/util/TreeSet.html
http://www.w3resource.com/java-tutorial/java-treeset.php
java collections
原文：http://www.cnblogs.com/zyip/p/4312764.html
