Last Stone Weight

You are given an array stones where stones[i] is the positive weight of the i-th stone.

Repeat the following step while at least two stones remain:

1. Take the two heaviest stones, with weights x <= y.
2. Smash them together:

• If x == y, both stones are destroyed.
• If x < y, the lighter stone is destroyed and the heavier one survives with new weight y - x. It goes back into the pile and can be chosen again.

When fewer than two stones remain, return the weight of the last stone, or 0 if none are left.