public class Codec {
    // Encodes a tree to a single string.
    public String serialize(TreeNode root) {
        test = root;
        String result = "[";
        if (root == null) return result + "]";

        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 += "null,";
                } else {
                    result += curr.val + ",";
                    q.add(curr.left);
                    q.add(curr.right);
                }
            }
        }

        int firstIdx = result.length() - 5;
        while (result.substring(firstIdx, result.length()).equals("null,")) {
            firstIdx -= 5;
        }
        return result.substring(0, firstIdx - 1) + "]";
    }

    //[1,2,null,null,
    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        return test;
    }
}