internal path length of complete binary tree

Question

can someone pls explain how is the internal path length of complete binary tree is O(n logn)? (if it is correct)

Comments

No. of Edges between a non-root node and root node in a tree is path length.

The sum of the path lengths of a tree's internal nodes is called the internal path length.

There are  'n/2' [ceil value] internal nodes approximately.

To find the distance between internal node and root node in a complete binary tree , worst case time be O(log n). 

for n/2 nodes, Worst case time be O(n * log n)

please correct me if i am wrong.

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any reference link for above topic???

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There is reference given in corme, search in end pages about internal path length or internet