code: private List<DependencyNode> levelTraverseTreeList(List<DependencyNode>  nodeList, List <DependencyNode> aggDepNodeList )
    {
      aggDepNodeList.addAll(nodeList);
      List<DependencyNode> NodeChildList = new ArrayList<DependencyNode>();
      for (DependencyNode node : nodeList) {
        if ( (node != null) && (node.getChildren() != null) ) {
          NodeChildList.addAll(node.getChildren());
        }
      }
  
      return NodeChildList;
    }
docs: helper function for traversing nodes
  in Level-Order way (also called BFS algorithm)