How can building a heap be O(n) time complexity?

Question

Can someone help explain how can building a heap be O(n) complexity?

Inserting an item into a heap is O(log n), and the insert is repeated n/2 times (t

Answer 1

"The linear time bound of build Heap, can be shown by computing the sum of the heights of all the nodes in the heap, which is the maximum number of dashed lines. For the perfect binary tree of height h containing N = 2^(h+1) – 1 nodes, the sum of the heights of the nodes is N – H – 1. Thus it is O(N)."