Set的实现是通过２叉排序树来实现，就是将所有的元素用一个树来存储，根元素大于左子树的元素，小于右子树的元素，所有的操作都是基于这个树，通过判断元素的大小来选择对左子树操作还是右子树操作，这样操作数量和树的层数成正比-Set by the realization of two binary sort tree to achieve, that is, all the elements to store a tree, the root element is greater than the le