public void construct()
	{
		len = graph.length;
		
		//put vertex that are not pointed to in the list
		int sum = 0;
		for(int i = 0; i < len; i++){
			for(int j =0; j < len; j++)
			{
				sum += graph[j][i];
			}

			if(sum == 0){
				list.add(i);
			}
			sum = 0;
		}
	}