search_dir_circle

代码说明：

  深度有向寻圈。访问当前某个顶点的后继，再访问后继的某个子顶点，若没有，返回上一层访问其他后继(Depth of the search circle. The current follow-up visit to a vertex, then visit a subsequent sub-vertex, if not, return to the level of other follow-up visit)