Anthropic·Software Engineer·Technical Phone Screen·Senior

Senior

May 2026

Summary

Interviewed for a software engineering role at Anthropic and got a fairly deep algorithmic problem involving execution logs and call stacks. The question had a lot of moving parts and the constraints were pretty specific, which made it feel like more than just a typical coding screen.

Questions Asked (1)

Given a single-threaded program's execution log (a list of timestamped BEGIN/END events for function IDs), compute the exclusive CPU time per function (not counting time spent in child calls), and also reconstruct which functions are on the call stack at a given query timestamp. The solution should run in O(n) time and use O(m + call depth) space. You also need to explain how you handle equal timestamps (END before BEGIN), half-open intervals, and edge cases like back-to-back events or a query time outside any active interval.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

This one took me a minute to even parse fully.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the event format and timestamp semantics, then propose a stack-based single-pass algorithm that tracks the current call stack and accumulates exclusive time by subtracting child time from total time. For stack reconstruction at a query time, either precompute intervals or use binary search on the event list, ensuring O(n) preprocessing and O(log n) query time.

Pro tip: Explicitly state your assumptions about timestamp ordering (e.g., END before BEGIN at equal timestamps) and half-open intervals, and mention that you would validate them with the interviewer before coding. This shows attention to detail and prevents miscommunication.

1. Clarify requirements and edge cases

Ask about event format, timestamp resolution, tie-breaking rules, and whether queries are interleaved or offline. Confirm that exclusive time excludes child calls and that intervals are half-open [start, end).

2. Design stack-based exclusive time computation

Use a stack to track active functions. On BEGIN, push the function and record its start time; on END, pop, compute total elapsed time, subtract time spent in children (tracked via a separate array or by maintaining child time on the stack), and add to the function's exclusive time.

3. Handle equal timestamps and half-open intervals

Process END events before BEGIN events at the same timestamp to ensure zero-duration intervals are handled correctly. Treat intervals as half-open: [start, end) so that back-to-back events don't overlap.

4. Reconstruct call stack at query time

Preprocess events into a list of intervals with associated stack snapshots, or use binary search to find the active interval containing the query time. For O(1) query, store stack state at each event and use binary search to locate the relevant event.

5. Analyze complexity and edge cases

Confirm O(n) time for preprocessing and O(m + depth) space, where m is number of functions. Discuss edge cases: query before first event, after last event, during nested calls, and at exact boundaries.

Key Points to Mention

Stack-based single-pass algorithm for exclusive time computation
Handling of equal timestamps: END before BEGIN to avoid negative durations
Half-open intervals [start, end) to correctly handle back-to-back events
Space complexity: O(m + call depth) where m is number of functions
Binary search or precomputed snapshots for O(log n) or O(1) stack queries
Edge cases: query outside any interval, nested calls, and zero-duration functions

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