如何在java中为二叉树添加索引？

import java.util.HashMap;

public class BinaryTreeIndexer {
    private HashMap<Integer, Integer> indexMap;

    public BinaryTreeIndexer() {
        indexMap = new HashMap<>();
    }

    public void addIndex(TreeNode root) {
        addIndexHelper(root, 0);
    }

    private void addIndexHelper(TreeNode node, int index) {
        if (node == null) {
            return;
        }

        indexMap.put(node.val, index);

        addIndexHelper(node.left, 2 * index + 1);
        addIndexHelper(node.right, 2 * index + 2);
    }

    public int getIndex(int value) {
        return indexMap.getOrDefault(value, -1);
    }
}

public class Main {
    public static void main(String[] args) {
        // 创建二叉树
        TreeNode root = new TreeNode(1);
        root.left = new TreeNode(2);
        root.right = new TreeNode(3);
        root.left.left = new TreeNode(4);
        root.left.right = new TreeNode(5);

        // 创建索引器并添加索引
        BinaryTreeIndexer indexer = new BinaryTreeIndexer();
        indexer.addIndex(root);

        // 获取节点值为4的索引
        int index = indexer.getIndex(4);
        System.out.println("Index of value 4: " + index);
    }
}