binary-indexed-tree

Binary Indexed Trees (a.k.a. Fenwick Trees).

Binary indexed trees are a data structure on indexes 1 through n. They allow you to compute the sum of all values at indexes 1 through i in O(logn) and to increase the value at index i in O(logn).

This implements binary indexed trees, both as an immutable data structure in pure code and as a mutable data structure using the ST Monad.

Both the immutable and mutable version have the same runtime complexity, but the mutable version has a smaller constant.

Written by Maxwell Sayles (2012).