An <a href="https://en.wikipedia.org/wiki/AVL_Tree" class="extiw" title="en:AVL Tree">en:AVL Tree</a>. Note that a deletion of node 4 requires tow rotations to rebalance the tree (right(3) +  left(5)).
<a href="//commons.wikimedia.org/w/index.php?title=User:HenHei&amp;action=edit&amp;redlink=1" class="new" title="User:HenHei (page does not exist)">Henning.H</a>, SVG graphic by <a href="//commons.wikimedia.org/w/index.php?title=User:Sim0nx&amp;action=edit&amp;redlink=1" class="new" title="User:Sim0nx (page does not exist)">Sim0nx</a> and  <a href="//commons.wikimedia.org/wiki/User:Reinhard_Kraasch" title="User:Reinhard Kraasch">Reinhard Kraasch</a>
CC BY-SA 2.5
created with <a href="https://en.wikipedia.org/wiki/Dia" class="extiw" title="en:Dia">en:Dia</a> and modified with <a href="https://en.wikipedia.org/wiki/Inkscape" class="extiw" title="en:Inkscape">en:Inkscape</a>