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Interview report

This is another candidate's assessment. The feedback and recommendations apply to their performance.

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Problem
Flight Log
Language
Python
Ended

Interview result

Mixed signal
The candidate delivered a correct, readable implementation that passed all 11 tests, using per-sensor histories and rightmost binary search. However, the interviewer introduced the key idea of separating sensor histories, and the candidate provided only a brief explanation of appending and binary search afterward. They could not explain why retrieval returns the most recent valid reading and voluntarily ended despite substantial remaining time. The implementation signal is strong, but the reasoning, requirement analysis, complexity analysis, and communication signals are materially weaker.
14/25

Total score

5 of 5 criteria scored

Strengths

  • Continue writing code with clear structure and localized responsibilities. The final record and retrieve methods were concise, readable, and handled the search boundaries correctly.
  • The candidate correctly identified the main operational costs as constant-time appending and logarithmic lookup.
  • The final implementation retained the latest valid binary-search candidate while continuing toward later timestamps, and it passed all 11 tests.

Focus next

  • Explain the requirements before coding, especially the meaning of “largest timestamp less than or equal to” and the behavior when no exact match exists.
  • Derive and justify load-bearing design decisions independently. The per-sensor grouping was supplied by the interviewer, and the candidate did not explain why global timestamp ordering keeps each sensor history sorted.
  • State complete complexity bounds, including total storage, and connect each bound to the relevant data structure and operation.
  • Make the binary-search invariant explicit: when a midpoint is valid, preserve it as the best answer so far and search right for a more recent valid reading.
  • Remain engaged long enough to explain and verify the solution. When directly asked about retrieval correctness, the candidate said they did not know and ended the interview.

Targeted recommendations

Based on the candidate's interview and score breakdown.

Practice problems

  1. Depth GaugeEasyUnlock problem
  2. Heartbeat MonitorEasyUnlock problem
  3. Luggage GroupingMediumUnlock problem
View Binary Search practice roadmap

Course lessons

Binary SearchCore conceptUnlock lessonThe First Five MinutesInterview executionUnlock lessonThink Out LoudInterview executionUnlock lesson

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Detailed assessment

Score breakdown

2/5Problem understanding

Assessment

The correct implementation reflects some practical understanding of the required behavior, but the candidate did not explain the requirements, relevant guarantees, or edge cases. When asked how the solution ensures the most recent value at or before a nonmatching query timestamp, they said they did not know and ended the interview. They should explicitly translate this requirement into a rightmost-valid-position search and connect the input guarantees to safe boundary handling.
3/5Algorithm and data structure

Assessment

The candidate ultimately named appending followed by binary search as the approach. This is suitable, and the final code implements it correctly. However, the interviewer first supplied the load-bearing premise of keeping each sensor's readings separate. The candidate did not independently justify why the histories remain ordered, why binary search applies, or why moving right after a valid midpoint finds the most recent qualifying entry. They should derive those connections and state the search invariant rather than relying primarily on familiarity with the pattern.
5/5Implementation quality

Assessment

The final code is correct, compact, and readily understandable. It uses a dictionary of chronological (timestamp, value) lists, clear local names such as readings, left, right, and mid, and comments that accurately describe the data organization and search behavior. The binary-search boundaries are consistent, initialization is safe under the stated valid-query guarantee, and all 11 tests passed. No implementation defect required correction.
2/5Complexity analysis

Assessment

The candidate correctly stated constant-time insertion and logarithmic lookup. They did not qualify insertion as amortized, define lookup in terms of one sensor's history, analyze total space, or justify the bounds from the dictionary, list append, and binary search operations. They should give the complete result: amortized O(1) recording, O(log k) retrieval for k readings of the queried sensor, and O(r) total storage for r records.
2/5Communication

Assessment

The candidate made a small portion of the technical plan visible by saying the solution appends records and performs binary search. Most material reasoning remained unspoken, including data organization, ordering, search correctness, assumptions, and tradeoffs. After a clear request to explain the non-exact-match case, they declined to continue. They should narrate the key decisions and invariants as they work so the interviewer can evaluate understanding independently of passing tests.

Interview materials

Problem and final code

Problem description

A spacecraft's telemetry system records sensor readings over time. Each reading consists of a sensor name, a value, and a timestamp. The system must support two operations:

  • record(sensor, value, timestamp) - Store a sensor reading at the given timestamp
  • retrieve(sensor, timestamp) - Return the value recorded for the sensor at the largest timestamp less than or equal to the query timestamp. The query timestamp may not match any recorded timestamp exactly, so return the most recent recorded value at or before the query time.

Timestamps are strictly increasing: each record operation uses a timestamp exactly 1 greater than the previous operation. The retrieve operation will always query for a sensor that has been recorded, and the query timestamp will always be at or after the sensor's first recorded timestamp.

Implement the FlightLog class with these operations.

Example

Input:
record altitude 35000 1
record speed 450 2
retrieve altitude 2
record altitude 36000 3
retrieve altitude 3
retrieve altitude 2

Output:
35000
36000
35000

Explanation:
- record altitude 35000 at t=1
- record speed 450 at t=2
- retrieve altitude at t=2: returns "35000" (no altitude at t=2, so return most recent from t=1)
- record altitude 36000 at t=3
- retrieve altitude at t=3: returns "36000"
- retrieve altitude at t=2: returns "35000" (still the most recent at or before t=2)

Constraints

  • 1 <= sensor.length <= 10^5
  • 1 <= value.length <= 50
  • 1 <= timestamp <= 10^5
  • At most 10^5 total operations
  • record timestamps are strictly increasing by exactly 1 each call
  • retrieve will always query a valid sensor at a valid timestamp
Final code

Conversation record

Interview transcript

Review the complete written conversation from this interview.