层序遍历

 // Encodes a tree to a single string.
    public String serialize(TreeNode root) {
        StringBuilder sb = new StringBuilder();
        sb.append("[");
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while (!queue.isEmpty()) {
            int size = queue.size();
            for (int i = 0; i < size; i++) {
                TreeNode node = queue.poll();
                sb.append(node == null ? "null," : node.val + ",");
                if (node != null) {
                    queue.offer(node.left);
                    queue.offer(node.right);
                }
            }
        }
        sb.deleteCharAt(sb.length() - 1);
        sb.append("]");
        return sb.toString();
    }
    // Decodes your encoded data to tree.
    public TreeNode deserialize(String data) {
        String[] str = data.substring(1, data.length() - 1).split(",");
        int index = 0;
        TreeNode root = generatorNode(str[index++]);
        Queue<TreeNode> queue = new LinkedList<>();
        if (root != null) {
            queue.offer(root);
        }
        while (!queue.isEmpty()) {
            TreeNode node = queue.poll();
            node.left = generatorNode(index == str.length ? "null" : str[index++]);
            node.right = generatorNode(index == str.length ? "null" : str[index++]);
            if (node.left != null) {
                queue.offer(node.left);
            }
            if (node.right != null) {
                queue.offer(node.right);
            }
        }
        return root;
    }
    private TreeNode generatorNode(String s) {
        if (s.equals("null")) {
            return null;
        }
        return new TreeNode(Integer.parseInt(s));
    }