Sets 元素不重复
HashSet<T>和SortedSet<T>
1)both implement the interface ISet<T> 实现接口
2)HashSet<T> contains an unordered list of distinct items 元素无序且不重复
SortedSet<T> the list is ordered 有序
1)both implement the interface ISet<T> 实现接口
2)HashSet<T> contains an unordered list of distinct items 元素无序且不重复
SortedSet<T> the list is ordered 有序