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Decagon·AI Engineer·Technical Phone Screen·Intermediate

Intermediate
May 2026

Summary

Interviewed for an AI Engineer role at Decagon and got a coding problem involving function call logs and exclusive execution time. Pretty standard stack-based problem but the nested call tracking tripped me up a bit.

Questions Asked (1)

Q1

Given a list of function call logs on a single-threaded CPU, where each log contains a function ID, whether it's a start or end event, and a timestamp, compute the exclusive execution time for each function. Nested calls should not count toward the caller's time. Return an array indexed by function ID.

Algorithms & Data Structures
Author's notes

My first instinct was to just accumulate raw time per function and I completely forgot to subtract nested call durations.

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AI HintsAI Generated

Suggested Approach

Use a stack to track active function calls, recording the start time and accumulated child time for each. When an end event occurs, compute the exclusive time as (end_timestamp - start_timestamp + 1) minus the accumulated child time, then add this exclusive time to the function's total and update the parent's accumulated child time. Finally, return the array of exclusive times indexed by function ID.

Pro tip: Clarify the timestamp semantics upfront: whether the end timestamp is inclusive (e.g., start at 0, end at 2 means 3 units) or exclusive. This off-by-one detail is a common pitfall and shows attention to detail.

1. Clarify assumptions and edge cases

Confirm timestamp inclusivity, input format, and constraints (e.g., function IDs range, log ordering). Ask about nested calls and whether logs are guaranteed valid.

2. Choose data structures

Use a stack to manage nested calls and an array to accumulate exclusive times. Each stack frame stores function ID, start time, and accumulated child time.

3. Process logs sequentially

For a start event, push a new frame onto the stack. For an end event, pop the top frame, compute exclusive time, update the function's total, and if the stack is not empty, add the exclusive time to the parent's accumulated child time.

4. Compute exclusive time correctly

Exclusive time = (end_timestamp - start_timestamp + 1) - accumulated_child_time. Ensure the +1 is applied if timestamps are inclusive.

5. Return and verify

Return the array of exclusive times. Walk through a simple example to verify correctness, especially for nested calls.

Key Points to Mention

  • Stack-based approach for handling nested function calls
  • Accumulating child time to subtract from parent's total time
  • Timestamp inclusivity and off-by-one handling
  • Time complexity O(n) and space complexity O(n) where n is number of logs
  • Edge cases: multiple top-level calls, deep nesting, zero-duration functions
  • Array indexed by function ID for output

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