I got an error on #8 after rejudging the problem. First i got AC.
This time I tried a brute force approach.
first i generated al binary digits from 0 to 2^N.
then i counted sum of the stones where the bit was 1.
The difference total-2*sum had to be minimal.

AC in 0.2 sec

I tried this approach but got TLE for test 3. Question i keep asking myself is, "Must you check all the 2^N combinations. if "No" how should you set your break point?

Try to write more effective code, don't use string and set at all!

Thank You. Just nailed it.