http://kor.i2p/books/The%20C%20Programming%20Language%20by%20K&R/chapter6.html
If
there is no child in the required direction, the new word is not in the tree,
and in fact the empty slot is the proper place to add the new word. This
process is recursive, since the search from any node uses a search from one
of its children. Accordingly, recursive routines for insertion and printing
will be most natural.