private List<Integer> intersection(List< ArrayList<Integer>> queryList) {
		
//		List<Integer> interResult = new ArrayList<Integer>();
		if(queryList == null || queryList.size() < 1)
			return null;
		
		List<Integer> interResult = queryList.get(0);
		
		List<Integer> commonDocs = new ArrayList<Integer>();
		for(int i =1; i < queryList.size(); ++i) {
			List<Integer> docs = queryList.get(i);
			for(Integer docId: docs) {
				if(interResult.contains(docId)) {
					commonDocs.add(docId);
				}
			}
			
			List<Integer> temp = interResult;
			interResult = commonDocs;
			commonDocs = temp;
			commonDocs.clear();
		}
		
		return interResult;
	}