Last Stone - Implementation

Here's the solution:

function lastStoneWeight(stones)
    heap := max-heap containing all stones

    while size of heap > 1
        y := extract max from heap
        x := extract max from heap
        if y > x
            insert (y - x) into heap

    if heap is empty
        return 0
    return extract max from heap

Time: $O(n \log n)$. Space: $O(n)$.