In a phrase of the problem statement it is said: "The state of objects that do not have any nested objects will be '-'.". From this I understood that when k = 0, s would necessarly be '-', but I then submitted this code, identical to the AC one, with one exception: if s = '+' and k = 0, then it would give TLE, which indeed happend but shouldn't.
Did I misinterpeted the statement or is the problem indeed wrong? In the latter case I will try to contact the author.
Thanks a lot in advance!