clrs; exercise 10.4.1 - draw a binary tree
clrs; exercise 10.4.2 - recursive binary tree walk
clrs; exercise 10.4.3 - nonrecursive binary tree walk
clrs; exercise 10.4.4 - left-child/right-sibling walk
clrs; exercise 10.4.5 - O(1) space binary tree walk
clrs; exercise 10.4.6 - left-child/right-sibl w/ 2 pointers & 1 bool