Determine whether a given binary tree is fully populated, where "fully populated" means that every internal node has exactly two children, and all terminal nodes are at the same depth.
Sigiloso
When I started solving this one, I briefly considered (and verbalized) the idea of just counting the nodes in the tree, determining the height, and checking to see if node_count == 2^height - 1. In retrospect, that would have been a slick solution. The actual solution I implemented was recursive; note that a fully populated binary tree has two fully populated subtrees as children. You also have to check whether the subtrees are the same height.