Last Stone Weight

You have stones with positive weights. Each turn, take the two heaviest stones x ≥ y and smash them: if x = y both are destroyed; otherwise the y stone is destroyed and the other becomes x − y. Continue until at most one stone is left. Print its weight, or 0 if none is left.

Input:

• line 1: n
• line 2: n weights

Output: the weight of the last stone, or 0.

Constraints: 1 ≤ n ≤ 10⁵, 1 ≤ weight ≤ 10⁹.

Think about it: sorting every turn is O(n² log n). A max-heap gives the two heaviest in O(log n) each. (Python's heapq is a min-heap — push negated weights.)