NIL-node when tree height 0 resp. subtree when &gt;0.
<a href="//commons.wikimedia.org/wiki/User:Nomen4Omen" class="mw-redirect" title="User:Nomen4Omen">Nomen4Omen</a>
2019-11-11
<span class="int-own-work" lang="en">Own work</span>