Q: Via Quora
Given the set of keys {6, 7, 1, 4, 2}, how many unique binary search trees of height 2 are possible? Draw the trees. Assume that the height of the root-node is zero.
A:
There are 6 unique Binary Search Trees (BST) of height 2 that can be constructed from the key set
$\{ 6, 7, 1, 4, 2 \}$ when all keys are used. Furthermore, there are 36 total BSTs of height 2 that can be constructed from said key set.