public static void sum(FeatureVector fv1, FeatureVector result){
        if (fv1==null||result==null) return;
        Iterator<Integer> it = fv1.getKey();
        while(it.hasNext()){
            int key = it.next();
            double sum = fv1.get(key) + result.get(key);
            if(sum!=0.0)
                result.add(key, sum);
            else
                result.remove(key);
        }
    }