Time Complexity of building a heap | GeeksforGeeks

BUILD-HEAP(A)       heapsize := size(A);       for i := floor(heapsize/2) downto 1           do HEAPIFY(A, i);       end for   END  

What is the worst case time complexity of the above algo?
Although the worst case complexity looks like O(nLogn), upper bound of time complexity is O(n). See following links for the proof of time complexity.

http://www.cse.iitk.ac.in/users/sbaswana/Courses/ESO211/heap.pdf/

http://www.cs.sfu.ca/CourseCentral/307/petra/2009/SLN_2.pdf

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