Fenwick Tree is a data structure that was described in a paper by Peter M. Feniwick in 1994. It is useful for efficiently calculating sums and updating element values in an array.
https://www.hackerearth.com/practice/notes/binary-indexed-tree-or-fenwick-tree/
https://www.geeksforgeeks.org/binary-indexed-tree-or-fenwick-tree-2/
https://cp-algorithms.com/data_structures/fenwick.html
https://sites.google.com/site/centrulinfo1/materiale-video/algoritmi-video/arbori-indexati-binar