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Microsoft·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
Jun 2026

Summary

Microsoft SWE interview with a string parsing problem that looks straightforward until you start thinking about edge cases. The state machine discussion at the end was the part I wasn't prepared for.

Questions Asked (1)

Q1

Given a string of C++ source code, remove all comments (both single-line '//' and block '/* */' variants, including multi-line blocks) and drop any lines that become empty as a result. Comment markers inside string literals should be treated as regular characters.

Algorithms & Data StructuresTechnical Trade-offs
Author's notes

I started with a naive approach scanning for '//' and '/*' substrings and immediately ran into trouble with overlapping cases.

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AI HintsAI Generated

Suggested Approach

Clarify the requirements and edge cases first, then propose a single-pass state machine that tracks whether you are in normal code, a string literal, a single-line comment, or a block comment. Emphasize handling string literals correctly and discuss trade-offs between in-place modification and building a new string.

Pro tip: Mention that you would write unit tests covering tricky cases like comment markers inside strings, escaped quotes, and multi-line block comments before coding. This shows production-level rigor and prevents subtle bugs.

1. Clarify requirements and edge cases

Ask about string literal handling (including escapes), nested comments, line continuation, and whether to preserve original line breaks. Confirm that empty lines should be dropped.

2. Choose an approach and data structures

Decide between in-place modification (if mutable) or building a new string. Use a state machine with states: NORMAL, IN_STRING, IN_SINGLE_COMMENT, IN_BLOCK_COMMENT.

3. Design the state machine logic

Define transitions: in NORMAL, detect '//' or '/*' or '"'; in IN_STRING, handle escapes and closing quote; in comments, skip until end marker. Track line boundaries to drop empty lines.

4. Implement and handle edge cases

Write clean code with careful indexing. Handle escaped quotes, block comments spanning multiple lines, and ensure that comment markers inside strings are ignored.

5. Test and discuss trade-offs

Walk through test cases: simple comments, comments in strings, multi-line blocks, empty lines. Discuss time/space complexity and alternative approaches.

Key Points to Mention

  • State machine with states for normal code, string literals, single-line comments, and block comments.
  • Proper handling of escaped characters inside string literals (e.g., \" and \\).
  • Detection of comment markers only when not inside a string literal.
  • Tracking line boundaries to remove lines that become empty after comment removal.
  • Time complexity O(n) and space complexity O(n) for output (or O(1) extra if in-place).
  • Edge cases: nested block comments (not allowed in C++), comments at end of file, and CRLF line endings.

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