You are modifying copy so nothing will happen. On my solution just for the consistency I did not modify it but the code is less memory efficient though - that's the tradeoff.

Is modifying the input really best practice?

1

5 yo , but anyway .

Vectors have a reserve member function .

So , even little ineficient can be removed ... with the right situation for reserve of course :)

It's definitely a little more inefficient, but I don't think that the inefficiency is a huge deal here. However, that is something to keep in mind.

isn't this very inefficient? I mean, using push_back every iteration leads to multiple reallocation, instead wouldn't it be better to initialize output vector with known size n?

have a look at c++ algorithms. http://en.cppreference.com/w/cpp/algorithm

wrong
Assert::That(stray({1, 2, 2}), Equals(1));
Expected: equal to 1
Actual: 2

wrong