I just got into the same problem, and finally able to understand what output means,
basically if you encounter a dot ( . ) remove the last item from the stack and put it in the output variable
['9', '8', '7', '6', '5', '4', '3', '2', '1']
taking them one by one as a stack
output = "123456789"
lastly when nothing remains in stack it will replace empty with 0 what leads to @ and exit