Codec.java

import java.util.*;

public class Codec
{
    private static final String STUB = "#";
    private static final String DELIMITER = ",";

    public String serialize(TreeNode root)
    {
        StringBuilder sb = new StringBuilder();
        sb.append(root == null ? STUB : root.val).append(DELIMITER);
        sb.append(root == null ? "" : serialize(root.left));
        sb.append(root == null ? "" : serialize(root.right));
        return sb.toString();
    }

    public TreeNode deserialize(String data)
    {
        Queue<String> queue = new LinkedList<>();
        queue.addAll(Arrays.asList(data.split(DELIMITER)));
        return buildTree(queue);
    }

    private TreeNode buildTree(Queue<String> queue)
    {
        String node = queue.poll();
        if (node.equals(STUB))
        {
            return null;
        }
        TreeNode root = new TreeNode(Integer.parseInt(node));
        root.left = buildTree(queue);
        root.right = buildTree(queue);
        return root;
    }
}