Post work on tree building: one more optimization tweak

This minor change brings small speedup: it moves the lower bound of tree building time for 100 000 annotations on my iPhone 4 (2010) to `1,7 - 2` seconds compared to older `1,8 - 2` seconds.

Please, don't think I am a performance maniac, I am just preparing for algorithms interview on the next week (ugh, the fourth in a pipe) and these are the materials I am studying that inspired me to make this obvious improvement - nothing was changed in algorithm, it is just a bit more efficient usage of temporary storage.

P.S. I am waiting for your review. I would really like to show all these PR-sequence merged, so I could demonstrate it to the interview guys as something I've done. But if review of this will take a time - it is ok, I will find a way to explain all this anyway.

Thanks.