如何打印一棵树（Java）

import java.util.LinkedList;

/**
 * 需求：按层打印一棵树
 * 说明：树是保存在一个链表中
 * created by wangjunfu on 2017-05-25.
 */
public class TreeNode {
    String data;
    TreeNode parent;
    LinkedList<TreeNode> childlist;

    TreeNode() {
        data = null;
        childlist = new LinkedList();
        parent = null;
    }

    //递归显示并打印一棵树
    private static void displayTree(TreeNode f, int level) {

        String preStr = "";     // 打印前缀
        for (int i = 0; i < level; i++) {
            preStr += "    ";
        }

        for (int i = 0; i < f.childlist.size(); i++) {
            TreeNode t = f.childlist.get(i);
            System.out.println(preStr + "-" + t.data);

            if (!t.childlist.isEmpty()) {
                displayTree(t, level + 1);
            }
        }
    }
}