sierra·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Jun 2026

Summary

Interviewed for a software engineering role at Sierra and got a data structure design problem that looked deceptively straightforward at first glance. One round, coding focused.

Questions Asked (1)

Design an OrderedStream class that accepts n items tagged with unique 1-based IDs and values. The insert method should store the value at the given ID position and return the longest contiguous sequence of already-inserted values starting from the current pointer, advancing the pointer past whatever gets returned.

Algorithms & Data StructuresSystem Design

Author's notes

The pointer mechanic is where I tripped up initially.

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AI HintsAI Generated

Suggested Approach

Clarify the problem constraints and design a data structure that efficiently stores values by ID and tracks the next expected ID. Use an array or hash map for O(1) insertion and a pointer that advances while consecutive IDs are present, returning the contiguous sequence. Discuss time and space complexity, and consider edge cases like duplicate inserts or out-of-range IDs.

Pro tip: Mention that the amortized time per insert is O(1) because the pointer only moves forward, and each element is returned at most once. This shows you understand the efficiency beyond the worst-case per call.

1. Clarify requirements and constraints

Ask about input ranges, whether IDs are guaranteed unique and within 1..n, and if duplicate inserts are possible. Confirm the return type and behavior when no contiguous sequence exists.

2. Choose data structures

Select an array of size n+1 (or a hash map) to store values by ID, and maintain a pointer (nextId) initialized to 1. This allows O(1) access and updates.

3. Implement insert logic

Store the value at the given ID. Then, while the value at nextId is present, collect it and increment nextId. Return the collected list (which may be empty).

4. Analyze complexity and edge cases

Explain that each insert takes O(1) amortized time because the pointer advances at most n times total. Discuss handling of duplicate IDs, invalid IDs, and the case where no sequence is returned.

5. Test with examples

Walk through a small example (e.g., n=5, inserts in various orders) to demonstrate correctness and pointer advancement. Verify that the returned sequences are correct.

Key Points to Mention

Use an array (or hash map) for O(1) storage and retrieval by ID.
Maintain a pointer (nextId) that tracks the smallest ID not yet returned.
Amortized O(1) time per insert because the pointer only moves forward.
Space complexity O(n) for storing up to n values.
Handle edge cases: duplicate IDs, IDs outside 1..n, and empty returns.
Return a list (or stream) of values in contiguous order starting from the pointer.

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