VP trees are interesting (previously discussed here: <a href="https:&#x2F;&#x2F;news.ycombinator.com&#x2F;item?id=3304685" rel="nofollow">https:&#x2F;&#x2F;news.ycombinator.com&#x2F;item?id=3304685</a>), and there is also the MVP variant (multi vantage point trees -- 
<a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;MVP_tree" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;MVP_tree</a>):<p>Indexing Large Metric Spaces For Similarity Search Queries
<a href="http:&#x2F;&#x2F;citeseerx.ist.psu.edu&#x2F;viewdoc&#x2F;download?doi=10.1.1.43.7492&amp;rep=rep1&amp;type=pdf" rel="nofollow">http:&#x2F;&#x2F;citeseerx.ist.psu.edu&#x2F;viewdoc&#x2F;download?doi=10.1.1.43....</a>