如何使我的二叉树遍历方法返回Java中的字符串？

public class BinaryTreeTraversal {

    // 二叉树节点定义
    class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;

        TreeNode(int val) {
            this.val = val;
        }
    }

    // 二叉树前序遍历方法
    public String preorderTraversal(TreeNode root) {
        StringBuilder sb = new StringBuilder();
        preorder(root, sb);
        return sb.toString();
    }

    private void preorder(TreeNode root, StringBuilder sb) {
        if (root == null) {
            return;
        }
        sb.append(root.val).append(" "); // 节点值转换为字符串并添加到结果字符串中
        preorder(root.left, sb);
        preorder(root.right, sb);
    }

    // 二叉树中序遍历方法
    public String inorderTraversal(TreeNode root) {
        StringBuilder sb = new StringBuilder();
        inorder(root, sb);
        return sb.toString();
    }

    private void inorder(TreeNode root, StringBuilder sb) {
        if (root == null) {
            return;
        }
        inorder(root.left, sb);
        sb.append(root.val).append(" "); // 节点值转换为字符串并添加到结果字符串中
        inorder(root.right, sb);
    }

    // 二叉树后序遍历方法
    public String postorderTraversal(TreeNode root) {
        StringBuilder sb = new StringBuilder();
        postorder(root, sb);
        return sb.toString();
    }

    private void postorder(TreeNode root, StringBuilder sb) {
        if (root == null) {
            return;
        }
        postorder(root.left, sb);
        postorder(root.right, sb);
        sb.append(root.val).append(" "); // 节点值转换为字符串并添加到结果字符串中
    }
}

public class Main {
    public static void main(String[] args) {
        BinaryTreeTraversal binaryTreeTraversal = new BinaryTreeTraversal();

        // 构建二叉树
        BinaryTreeTraversal.TreeNode root = binaryTreeTraversal.new TreeNode(1);
        root.left = binaryTreeTraversal.new TreeNode(2);
        root.right = binaryTreeTraversal.new TreeNode(3);
        root.left.left = binaryTreeTraversal.new TreeNode(4);
        root.left.right = binaryTreeTraversal.new TreeNode(5);

        // 前序遍历
        String preorder = binaryTreeTraversal.preorderTraversal(root);
        System.out.println("Preorder traversal: " + preorder);

        // 中序遍历
        String inorder = binaryTreeTraversal.inorderTraversal(root);
        System.out.println("Inorder traversal: " + inorder);

        // 后序遍历
        String postorder = binaryTreeTraversal.postorderTraversal(root);
        System.out.println("Postorder traversal: " + postorder);
    }
}