Time Complexity building a heap(Optimised)

#1

i am not able to understand the explanation given in the video of how the complexity is O(N).
can you pls help me with it

#2

If you perform heapify operation in reverse order, the complexity decreases.
Refer this https://www.geeksforgeeks.org/building-heap-from-array/
Also watch the video once again as the same thing was explained there.

#3

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