i can not understand this realy

It's better than other because it does not allocate extra bytes, it uses same string.

Условие не понятно, не понятно как взаимодействовать с данной функцией, что и куда возвращать. Very bad explain of problem.

crazy!

Are you hacker?

Run only the first "Sample Tests" with in your code (just before your return):

std::cout << sum << "," << NOD << "," << sum / NOD << "," << grandDenom / NOD << std::endl;

and compare the expected result to your printing to see what is wrong in your code.
To avoid having a big gcd you should first simplify the fractions. On the other hand you could make this first calculation by hand to control your result.

Update: I added a small utility so that you can view your result and the expected result.

Expected: equal to [unsupported type]
Actual: [unsupported type] genius. FIX IT FOR C++

You really want sort over 99999 elements only for find minimum?

Are you crazy?