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.
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.
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