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Datadog·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
Apr 2026

Summary

Datadog SWE interview with a pretty gnarly streaming/matching problem. The kind of question where the core idea isn't too bad but the edge cases and efficiency requirements will eat you alive if you're not careful.

Questions Asked (1)

Q1

You're given a stream of strings, each prefixed with either 'Q:' (a query, which is a set of space-separated words) or 'L:' (a log line). Queries should be indexed as they arrive. For each log line received, output all previously seen queries where every word in that query appears at least once in the log line. Matching is case-sensitive and word boundaries are defined by spaces. How do you design and implement this efficiently for online processing?

Algorithms & Data StructuresSystem Design
Author's notes

My first instinct was to just iterate over all stored queries for each log and check word membership, which works but feels bad at scale.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and constraints, then propose an inverted index mapping each word to the set of query IDs containing it. For each log line, tokenize into words, retrieve candidate query IDs from the index, and intersect those sets to find queries whose every word appears in the log line.

Pro tip: Mention that you would optimize for the common case where log lines are much longer than queries, and that you can use bitsets for fast intersections when the number of queries is large.

1. Clarify requirements and constraints

Ask about expected scale (number of queries, log lines, words per query/line), latency requirements, and whether queries can be removed or updated. This informs data structure and algorithm choices.

2. Design the indexing structure

Propose an inverted index: a hash map from word to a set of query IDs that contain that word. Also maintain a list or map of query ID to its set of words for quick lookup and potential verification.

3. Process queries online

When a query arrives, tokenize it into words, assign a unique query ID, and for each word, add the query ID to the inverted index. Optionally, store the query's word set for later use.

4. Process log lines efficiently

For each log line, tokenize into words, deduplicate words, and collect candidate query IDs by looking up each word in the inverted index. Intersect these candidate sets to find queries where all words are present. Output the matching query IDs (or queries).

5. Optimize and handle edge cases

Discuss optimizations: using bitsets for fast intersections, caching frequent words, early termination if a query's word count exceeds log line word count, and handling case sensitivity and word boundaries as specified.

Key Points to Mention

  • Inverted index mapping words to query IDs
  • Set intersection to find queries with all words present
  • Time complexity: O(total words in log line + sum of sizes of candidate sets)
  • Space complexity: O(total unique words across all queries)
  • Optimization with bitsets for large-scale intersections
  • Handling case sensitivity and space-delimited tokenization

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