The ans is 2^{5}=32
Here the elements are 1,2,3,4,5,6 and we need only 1 leaf
1.So let us fix a root node: To fix the root node we have 2 choice either 1 or 6 from 1,2,3,4,5,6
(if we select any non-extreme elements then only 1 leaf is not possible)
Let us say we select 1 as root node
2.In each and every level we have 2 different option that is to select i,e; either of extreme elements
So in 2nd level we can select 2,3,4,5,6
Let us say we select 6 in 2nd level
3.We still have 2 option to select in 2,3,4,5
So this is true in all levels except last level since only 1 element is left.
Therefore, total possibilities are : 2 * 2 * 2 * 2 = 2^{4}
The same condition is true for 6 as a root
Hence, 2^{4}* 2 (1 as root or 6 as root) = 2^{5}