toMap with a Merge Function

- record Sale(String region, int amount) in Solution - static Map<String, Integer> totalsByRegion(List<Sale> sales) — sum amounts per region using Collectors.toMap with Integer::sum as the merge function. - static Map.Entry<String, Integer> topRegion(List<Sale> sales) — returns the region's entry with the highest total (ties: smallest region name wins). Duplicate keys without a merge function throw IllegalStateException — the tests make sure you used one.

Difficulty: intermediate