Heapsort is one of the fastest sorting algorithms. The best and the worst cases for heapsort have same $O(n\log(n))$ performance.
At first heapsort creates heap from data with
buildHeap function. Heap is organized in linear array as follows. Every $i$-th element has two children: $(2i)$-th element and $(2i+1)$-th one. The biggest element of the array is placed on the top of the heap.