[Recursion + Visitor Pattern] Realize the tree traversal processing of nodes

    /**
     * 把嵌套树结构 Tree 转成 Graph 图结构
     *
     * @param tree Tree
     * @return Graph
     */
    public static GraphVO tree2graph(ItemVO tree) {
        List<EdgeDO> edgeList = new ArrayList<>();
        buildEdges(tree, edgeList);
        final List<ItemVO> nodeList = new ArrayList<>();
        visitTree(tree, t -> {
            ItemVO node = new ItemVO();
            BeanUtils.copyProperties(t, node);
            // 图节点，children 置为空
            node.children = new ArrayList<>();
            nodeList.add(node);
        });
        GraphVO graph = new GraphVO();
        graph.setEdgeList(edgeList);
        graph.setItemList(nodeList);
        return graph;
    }


    /**
     * 访问者模式，递归遍历树节点
     *
     * @param t       树节点
     * @param visitor 访问者
     */
    private static void visitTree(ItemVO t, TreeVisitor visitor) {
        visitor.visit(t);
        if (null != t.children) {
            for (ItemVO child : t.children) {
                visitTree(child, visitor);
            }
        }
    }

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