Molocoads·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Apr 2026

Summary

Interviewed for a software engineer role at Molocoads and got a range sum query problem that started simple and escalated fast. They walked through both the immutable and mutable versions, which meant knowing two completely different approaches under pressure.

Questions Asked (1)

Given an integer array, implement two operations: update a value at a given index, and return the sum of elements between two indices inclusive. Walk through both an immutable and mutable version, and compare the data structures you'd use for each.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

Started with the prefix sum approach since that's the obvious entry point, O(1) query but falls apart the moment updates come into the picture.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the problem requirements and constraints, then present the immutable solution using prefix sums for O(1) range sum queries. For the mutable version, discuss trade-offs between a Fenwick tree (BIT) and a segment tree, explaining their time and space complexities. Conclude by comparing the data structures and justifying your choice based on expected operation frequencies.

Pro tip: Always discuss the trade-offs between update and query times, and mention that if updates are rare, a simple prefix sum with rebuild might be sufficient. This shows you consider practical scenarios beyond textbook solutions.

1. Clarify Requirements and Constraints

Ask about the frequency of updates vs. queries, the size of the array, and whether the array is static or dynamic. This determines the optimal data structure.

2. Immutable Version: Prefix Sums

Explain that for an immutable array, precompute a prefix sum array to answer range sum queries in O(1) time, with O(n) preprocessing and O(n) space.

3. Mutable Version: Fenwick Tree (BIT)

Describe using a Fenwick tree to support point updates and prefix sum queries in O(log n) time, with O(n) space. Show how to compute range sums as difference of prefix sums.

4. Alternative: Segment Tree

Mention segment tree as an alternative that also gives O(log n) for both operations, with more flexibility for other range queries, but higher constant factors and space.

5. Compare and Conclude

Compare the data structures: prefix sums for static arrays, Fenwick tree for balanced update/query, segment tree for complex queries. Justify your recommendation based on the clarified requirements.

Key Points to Mention

Time complexity: O(1) query for immutable, O(log n) for mutable with BIT/segment tree.
Space complexity: O(n) for all approaches.
Fenwick tree is simpler and more space-efficient than segment tree for point updates and range sums.
Segment tree can handle more complex range queries (e.g., min, max) but has higher constant factors.
Prefix sum array requires O(n) update if array is mutable, which is inefficient for frequent updates.
Trade-offs: choose based on update/query frequency and whether additional operations are needed.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.