Doubt : prev tifr question on GO

Question

https://gateoverflow.in/18749/tifr2010-b-26 

1.In this question,  if 'a' min element and  'b' is max element in given tree then "n-1" additions will be required, then how "logn" additions ?

2. We can do it by finding inorder traversal in O(n) time but above solution from link is optimal i guess as its taking O(logn) but i am not getting it. 

Can someone explain both points 1 and 2.

Comments

HeadShot

Just tell me how such nodes will be whose values lies between    a<=x <=b in the Balanced binary tree ???

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@kumar.dilip 

say, 1 to 100 elements are there in binary search tree then let a= 1 and b= 100 as worst case possibility.

so to add 1 to 100 we need 99 additions.