拓扑排序的结果不是唯一的，试写出下图任意2个不同的拓扑序列。

A.1

B.2

C.3

D.4

A.1

B.2

C.3

D.4

有向图G如下图所示，请写出所有拓扑排序序列。所有的顶点都直接用其数字标号表示，如拓扑排序序列为，那么请写成1234（中间没有空格）。不同的拓扑排序序列按照字典序排序，中间用一个空格隔开。 Directed graph G looks like following graph, please list all the topological order sequences. All the vertices are marked by numbers directly. Like topological order sequence V1V2V3V4, we write it as 1234(with no blank space).Different topological order sequences are sorted according to alphabet order, and separated by a blank space.