Sword refers to Offer-37 serialized binary tree

public static String serialize(TreeNode root) {
    if (root == null){
        return "[]";
    }
    // 不涉及线程安全 StringBuilder更快
    StringBuilder res = new StringBuilder("[");
    Queue<TreeNode> queue = new LinkedList<>();
    queue.add(root);
    // 层序遍历
    while (!queue.isEmpty()){
        TreeNode t = queue.poll();
        // 当前节点不为空 则追加值 并把左右节点入队
        if (t != null){
            res.append(t.val + ",");
            queue.add(t.left);
            queue.add(t.right);
        }else {
            // 反之追加null
            res.append("null,");
        }
    }
    res.deleteCharAt(res.length() - 1);
    res.append("]");
    return res.toString();
}

public static TreeNode deserialize(String data) {
    if ("[]".equals(data)){
        return null;
    }
    String[] array = data.substring(1, data.length() - 1).split(",");
    TreeNode root = new TreeNode(Integer.parseInt(array[0]));
    Queue<TreeNode> queue = new LinkedList<>();
    queue.add(root);
    int i = 1;
    // 循环每次都把当前结点的左右孩子加上
    while (!queue.isEmpty()){
        TreeNode node = queue.poll();
        if(!array[i].equals("null")) {
            node.left = new TreeNode(Integer.parseInt(array[i]));
            queue.add(node.left);
        }
        i++;
        if(!array[i].equals("null")) {
            node.right = new TreeNode(Integer.parseInt(array[i]));
            queue.add(node.right);
        }
        i++;
    }
    return root;
}