Method checkValid

ZezeJava/ZezeJava/src/main/java/Zeze/Collections/DAG.java:48–51

()

	}

	public void checkValid() throws Exception {
		if (!isValid())
			throw new Exception("DAG is invalid."); // 让异常提示更加智能
	}

	public boolean isEmpty() {
		return graph.vertexSet().isEmpty();