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Molocoads·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say
Jun 2026

Summary

Interviewed for a software engineer role at Molocoads and got a range-sum problem that started simple and then got progressively harder. The follow-up about making it mutable is where things got interesting and I felt a bit underprepared for the full tradeoffs discussion.

Questions Asked (1)

Q1

Design a data structure that supports efficient range sum queries on an integer array, first for an immutable array, then extended to support point updates as well. Both operations should scale to arrays and query counts in the hundreds of thousands.

Algorithms & Data StructuresTechnical Trade-offsSystem Design
Author's notes

Part one was fine, prefix sums, done in two minutes.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the requirements: immutable array first, then point updates. For the immutable case, propose a prefix sum array for O(1) queries. For the dynamic case, introduce a Fenwick tree (Binary Indexed Tree) or segment tree, explaining their O(log n) update and query times, and discuss trade-offs.

Pro tip: Mention that Fenwick trees are simpler and more memory-efficient for point updates and range sums, but segment trees are more flexible for other operations. Also, note that if updates are infrequent, a sqrt decomposition could be a simpler alternative.

1. Clarify Requirements

Confirm that the array is initially immutable, then extended to support point updates. Ask about the expected frequency of updates vs. queries and memory constraints.

2. Immutable Solution

Propose a prefix sum array: precompute cumulative sums so that range sum queries are O(1). Mention that this is optimal for static arrays.

3. Dynamic Solution

Introduce a Fenwick tree (BIT) or segment tree. Explain that both support point updates and range sum queries in O(log n) time, which scales to hundreds of thousands of operations.

4. Compare and Choose

Discuss trade-offs: Fenwick trees are simpler, use less memory, and are faster in practice for this specific problem; segment trees are more versatile for other range operations.

5. Analyze Complexity

Summarize time and space complexities: prefix sum O(n) build, O(1) query; Fenwick/segment tree O(n) build, O(log n) update/query, O(n) space.

Key Points to Mention

  • Prefix sum array for immutable arrays: O(1) range sum queries.
  • Fenwick tree (Binary Indexed Tree) for dynamic arrays: O(log n) point updates and range sum queries.
  • Segment tree as an alternative: O(log n) operations, more flexible but more complex.
  • Trade-offs: Fenwick tree is simpler and more memory-efficient; segment tree supports more operations (e.g., range updates, min/max).
  • Time and space complexity analysis for each approach.
  • Scalability: both Fenwick and segment trees handle hundreds of thousands of operations efficiently.

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