HeapifyDown.swift

mutating private func heapifyDown() {
  var index = 0
  while hasLeftChild(index) {
      var smallerChildIndex = getLeftChildIndex(index)
      if hasRightChild(index) && rightChild(index) < leftChild(index) {
          smallerChildIndex = getRightChildIndex(index)
      }

      if items[index] < items[smallerChildIndex] {
          break
      } else {
          swap(indexOne: index, indexTwo: smallerChildIndex)
      }
      index = smallerChildIndex
  }
}