Section Recap

You learned small-to-large merging, a technique that combines child sets in $O(n \log n)$ time instead of $O(n^2)$. You saw the proof: each element moves at most $\log n$ times because the set size doubles with each move.

You solved problems requiring distinct counts and frequency tracking. You know when to use this technique: when you need to merge containers from children into the parent, not add numbers.