ascending  : 4 5 6 10 11 13 15 16 
descending : 16 15 13 11 10 6 5 4 
iterative  : 16 15 13 11 10 6 5 4 
tree height h = 3, deepest recursive call = 4
single node: 42 
empty tree : (nothing)
chain 1..6 : 6 5 4 3 2 1 
