Smash the Heaviest Stones

stones lists the weights of some stones. Each weight is a positive whole number. Repeat these steps:

1. Pick the two heaviest stones. Call the lighter weight x and the heavier one y, so x <= y.
2. If x === y, remove both.
3. If not, remove both and put back one stone that weighs y - x.

Stop when one stone or none is left. Return the weight of the stone that is left, or 0 if none is.

• stones holds between 1 and 1,000 stones.
• Each weight is between 1 and 1,000.
• stones may be in any order.