We live in an age of information explosion. The indexes of
search engine Google reached 1 trillion URLs at 2008 [1].
Face to the enormous web resources, about 30 % time of
intellectual work is spent on retrieval [2]. The experience
of using search engine tells us that we cannot retrieve and
perceive all search results, because in most cases, the
amount of these search results are too big to be processed
in time. Therefore, it has become an important issue for
more and more researchers to make the use of web
resources more efficient