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Two Sigma·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
May 2026

Summary

Two Sigma coding round, one problem the whole session. The question looked manageable at first glance but the edge cases around duplicate characters had me second-guessing myself more than I'd like to admit.

Questions Asked (1)

Q1

Given a string of length n, consider all n strings formed by deleting exactly one character. For each of those strings, enumerate every non-empty subsequence. Collect all results across all deletions, deduplicate, and return them sorted lexicographically.

Algorithms & Data Structures
Author's notes

My first instinct was to just brute-force the whole thing since n is capped at 15, which is tiny.

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AI HintsAI Generated

Suggested Approach

First, clarify the problem and constraints (e.g., string length, character set) to ensure alignment. Then, discuss a brute-force approach and its complexity, followed by an optimized strategy using a set to deduplicate and sorting at the end. Emphasize correctness and efficiency trade-offs.

Pro tip: Mention that the total number of subsequences across all deletions is O(n * 2^n), so deduplication is crucial; using a hash set avoids redundant storage and sorting can be done once at the end.

1. Clarify and Define

Confirm the problem details: string length, character set, whether empty subsequences are excluded, and expected output format. This ensures you solve the right problem.

2. Brute-Force Approach

Describe generating all n strings by deleting one character, then for each, enumerate all non-empty subsequences. Analyze time and space complexity: O(n * 2^n) subsequences.

3. Optimize with Deduplication

Use a hash set to collect subsequences, automatically removing duplicates. Discuss potential memory concerns and whether streaming or trie-based approaches could help.

4. Sort and Return

Convert the set to a list and sort lexicographically. If needed, discuss sorting complexity O(m log m) where m is the number of unique subsequences.

5. Test and Validate

Walk through a small example (e.g., 'abc') to verify the approach, and consider edge cases like empty string or repeated characters.

Key Points to Mention

  • Time and space complexity analysis, including the exponential number of subsequences.
  • Use of a hash set for deduplication to handle duplicates efficiently.
  • Lexicographical sorting and its complexity.
  • Edge cases: empty string, single character, all identical characters.
  • Potential optimizations: early pruning, trie for deduplication, or bitmask enumeration.
  • Clarifying questions to ask the interviewer before coding.

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