If you guys like visual-explanations that are a bit more intuitive -- I put actually made guides on both data-structures and algorithms not too long ago which you can find here:
The visual is extremely hard to understand how it relates to the description, given that it looks to be 1-indexed, but the description only makes sense for 0-indexing (4th element stores the sum of the first 4 elements), not to mention none of the binary indexes seem to be storing the correct sums (or they're storing sums of a tree that isn't being presented).
