Fenwick tree (Binary Indexed Tree): point updates and prefix sums in O(log n).

A Fenwick tree answers prefix-sum queries and point updates in O(log n) with a single flat array and one bit trick. The signal is the lowbit operation and why it beats a plain prefix array under updates. Here is the answer.

Updated Aug 2026 · Grounded in real Applied AI Engineer interview loops and written to a senior-engineer editorial bar.