tree_zig_zag - initial commit of hints

Break out the hints into a separate file so that people don't risk
reading spoilers. Hints should only be referred to if you attempt
the problem for a little bit.
tree_zig_zag - initial commit of empty human-readable solution
tree_zig_zag - README - fix format to allow BSTs

Need to be able to specify a null left child, so change the format.
tree_zig_zag - generator - initial commit

A script that can generate a random BST that is not balanced.