private void clustering() {
        Random rand = new Random(77);
        int sqrt_N = (int)Math.round(Math.sqrt(FREE_DOC_ID));
        for(int i = 0; i< sqrt_N; i++) {
            int k = rand.nextInt(FREE_DOC_ID);
            if(!leaders.contains(k))
                leaders.add(k);
            else sqrt_N++;
        }
        System.out.println("SQRT_N: " + leaders.size());
        findFollowers();
        
    }