14. The apparatus of claim 13 , wherein after the clustering unit generates the suffix set, the clustering unit stores the suffix set in the first storage unit and the second storage unit.
14. The apparatus of claim 13 , wherein after the clustering unit generates the suffix set, the clustering unit stores the suffix set in the first storage unit and the second storage unit. 15. The apparatus of claim 14 , wherein the clustering unit establishes the suffix set as a sub-tree.