Last Stone Weight

Write `last_stone_weight(stones)` simulating this: repeatedly take the two heaviest stones and smash them. Equal weights destroy both; otherwise the difference goes back into the pile. Return the last remaining weight, or `0`. [2,7,4,1,8,1] -> 1 Sorting after every smash is O(n^2 log n). A heap makes each round O(log n). Python only has a min-heap, so negate the values — and remember to negate back when you read them.