I think it checks extra nodes when (depth > length) and it's not necessary.

Yes, but the code will never attempt to read it. That statement is in the if-condition, preceded by the statement "i < roman.size() - 1 && (...)". If the previous statement is false, the if-condition is "short-circuited", meaning the second statement is ignored.

In the last i , roman[i+1] is out of index. isn't it?