one leaf node ==> it should be chain of nodes !
let denote the nodes with A,B,.....
A ------------- B ------------- C -------------- D ---------- E ------------- F
those links are either left or right ==> 2${^\text{no.of links}} = 2^5$ = 32 structures possible !
it is BST, So in each structure, you can place a unique value for the Node ==> 32*1 = 32