Method dfs

java/0606-construct-string-from-binary-tree.java:21–32

(TreeNode root, StringBuilder res)

        return res.toString().substring(1, res.length()-1);
    }
    private void dfs(TreeNode root, StringBuilder res){
        if(root == null)
            return;

        res.append("(");
        res.append(root.val);
        if(root.left == null && root.right != null)
            res.append("()");
        dfs(root.left, res);
        dfs(root.right, res);
        res.append(")");
    }
}