The Region Quadtree Demo from U-Maryland was invaluable in teaching me how PR Quadtrees worked. http://wiki.algoviz.org/AlgovizWiki/Viz/RegionQuadtree As a GTA for a data structures course last year, I had several students come to my office hours to inquire about the Union-Find structure, particularly the case where path compression is enabled. I didn’t really understand path compression until after looking at Virginia Tech’s Union-Find viz. http://wiki.algoviz.org/AlgovizWiki/Viz/VtUnionFind