@Override
	public boolean isAcyclic() {
		// TODO Auto-generated method stub
		Color[] color = new Color[size];
		for(int i = 0; i < size; ++i) {
			color[i] = Color.WHITE;
		}
		for(int i = 0; i < size; ++i) {
			if(color[i] == Color.BLACK) continue;
			if(color[i] == Color.WHITE && dfs(color, i)) continue;
			return false;
		}
		return true;
	}