4. The method of claim 3 , further comprising: determining a plurality of importance levels of the search key word units; determining, based at least on the plurality of importance levels of the search key word units, a plurality of current importance levels of the current search key word units; selecting among the current key word units selective current key word units whose importance levels satisfy a precondition; determining, using the plurality of search key word tables corresponding to the plurality of stages, category information that corresponds to the selective current key word unit as the current category information.
5. The method of claim 4 , wherein selecting among the current key word units selective current key word units whose importance levels satisfy a precondition that includes selecting current key word units whose importance levels meet a specified threshold.