helper functions

[jeremymanning]

it'd be nice to have some sample notebooks/code to help with debugging and solving these problems. for example:

• convert a list to a binary tree (and vice versa)
• convert a list to a linked list (and vice versa)
• visualize a binary tree
• define classes for some common data structures (linked list, binary tree)
• implement some classic algorithms-- tree search (BFS, DFS), find common ancestor in a tree, etc.

could add these as we go... 🤔