Other interesting recursive problems:
1) A "strictly binary tree" is a binary tree that has nonempty left
and right substrees. Write a C function that accepts the root of a
tree and returns true if the tree is strictly binary; otherwise,
return false.
2) A "complete binary tree of depth d" is a strictly binary tree all
of whose leafs are at level d. Write a recursive function that
accepts the root of a tree and returns true if the tree is complete;
otherwise, return false.
3) Write a recursive function to reverse the links of a singly
linked list.