heapsort2.swift

public var isEmpty: Bool {
    return elements.isEmpty
}

public var count: Int {
    return elements.count
}

public func leftChildIndex(forParentAt index: Int) -> Int {
    return 2 * index + 1
}

public func rightChildIndex(forParentAt index: Int) -> Int {
    return 2 * index + 2
}

public func parentIndex(forChildAt index: Int) -> Int {
    return (index - 1) / 2
}