Applied·Machine Learning Engineer·Technical Phone Screen·Intermediate

Intermediate

Jun 2026

Summary

Interviewed for an ML Engineer role at Applied and got a data structures implementation question, which surprised me a bit. Not what I expected going in, but it was a fair problem if you've thought about circular buffers before.

Questions Asked (1)

Design and implement a fixed-capacity double-ended queue (deque) from scratch. It should support insertions and deletions from both ends, plus peek and status checks, all in O(1) time. Handle the case where the deque is full or empty gracefully, and make sure wraparound works correctly.

Algorithms & Data StructuresSystem DesignTechnical Trade-offs

Author's notes

My first instinct was to use a plain array with two pointers, which is the right call, but I fumbled the wraparound logic for a bit.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the requirements and constraints, then describe a circular buffer implementation using a fixed-size array with head and tail indices. Explain how to handle edge cases like full/empty and wraparound using modulo arithmetic, and discuss time complexity for each operation. Finally, mention potential optimizations or trade-offs for ML engineering contexts.

Pro tip: Emphasize that in ML pipelines, deques are often used for sliding window computations, so O(1) operations and memory efficiency are critical; also mention that thread-safety might be needed in production, but keep the core implementation simple.

1. Clarify Requirements and Constraints

Ask about expected capacity, data types, thread-safety, and whether dynamic resizing is needed. Confirm that all operations must be O(1) and that the deque is fixed-capacity.

2. Choose Data Structure and Design

Propose a circular buffer using a fixed-size array with head and tail indices, and a size counter. Explain how wraparound works using modulo arithmetic.

3. Define Operations and Edge Cases

Detail insertFront, insertRear, deleteFront, deleteRear, peekFront, peekRear, isEmpty, isFull. Describe how to handle full/empty gracefully (e.g., return false or throw exception).

4. Analyze Complexity and Trade-offs

Confirm O(1) time for all operations and O(n) space. Discuss trade-offs: array-based vs linked list, and potential need for thread-safety in ML serving.

5. Implement and Test

Write clean code with comments, then walk through test cases: empty, full, wraparound, and alternating operations. Mention unit testing for edge cases.

Key Points to Mention

Circular buffer with head, tail, and size for O(1) operations
Modulo arithmetic for wraparound: (index + 1) % capacity
Graceful handling of full/empty: return boolean or throw exception
Time complexity: all operations O(1); space O(capacity)
Trade-offs: array vs linked list, thread-safety, and use in ML sliding windows
Edge cases: single element, full deque, empty deque, wraparound

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