如何将平面数据结构显示为分层数据结构(Java)？

class TreeNode {
    Object data;
    TreeNode parent;
    List<TreeNode> children;

    public TreeNode(Object data) {
        this.data = data;
        this.children = new ArrayList<>();
    }

    public void addChild(TreeNode child) {
        child.parent = this;
        children.add(child);
    }
}

class Tree {
    TreeNode root;

    public Tree(Object data) {
        this.root = new TreeNode(data);
    }

    public void addNode(Object parentData, Object childData) {
        TreeNode parentNode = findNode(parentData);
        if (parentNode != null) {
            TreeNode childNode = new TreeNode(childData);
            parentNode.addChild(childNode);
        }
    }

    public TreeNode findNode(Object data) {
        return findNode(root, data);
    }

    private TreeNode findNode(TreeNode node, Object data) {
        if (node.data.equals(data)) {
            return node;
        }
        for (TreeNode child : node.children) {
            TreeNode result = findNode(child, data);
            if (result != null) {
                return result;
            }
        }
        return null;
    }
}

public static Tree createTree(Map<Object, Object> flatData, Object rootData) {
    Tree tree = new Tree(rootData);
    for (Object childData : flatData.keySet()) {
        Object parentData = flatData.get(childData);
        tree.addNode(parentData, childData);
    }
    return tree;
}

public static void printTree(TreeNode node, int level) {
    for (int i = 0; i< level; i++) {
        System.out.print("\t");
    }
    System.out.println(node.data);
    for (TreeNode child : node.children) {
        printTree(child, level + 1);
    }
}

public static void main(String[] args) {
    Map<Object, Object> flatData = new HashMap<>();
    flatData.put("A", null);
    flatData.put("B", "A");
    flatData.put("C", "A");
    flatData.put("D", "B");
    flatData.put("E", "B");
    flatData.put("F", "C");

    Tree tree = createTree(flatData, "A");
    printTree(tree.root, 0);
}