The minimum height would occur when when the tree is symmetrical. That is when the left and right subtrees off the root are identical.
n(1) = 0 only node is the root; h(1)=0
n(2) = 3 root and "first" subtree; h(2)=1
n(3) = 7 each of the two "leaves" then gets two branches giving 4 more nodes; h(3) = 2
so for n(T) = 2k-1 what is h(T)?
If still can't get it comment back.
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