public class Codec {
    // Encodes a tree to a single string.
    public String serialize(TreeNode root) {
        List<String> result = new ArrayList<>();

        Queue<TreeNode> q = new LinkedList<>();

        q.add(root);

        while (!q.isEmpty()) {
            int preSize = q.size();

            for (int i = 0; i < preSize; i++) {
                TreeNode curr = q.poll();

                if (curr == null) {
                    result.add("null");
                } else {
                    result.add(String.valueOf(curr.val));
                    q.add(curr.left);
                    q.add(curr.right);
                }
            }
        }
        System.out.println(String.join(",", result));
        return String.join(",", result);
    }

    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        if (data == null) return null;

    }
}