LinkedIn·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Jul 2026

Summary

LinkedIn software engineering interview with a coding question focused on string manipulation. Pretty standard technical screen but the edge case requirements made it more involved than I expected.

Questions Asked (1)

Write a function to check if a string is a palindrome after stripping out non-alphanumeric characters and ignoring case. Include time and space complexity analysis, and write tests that cover edge cases like special characters and Unicode input.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

I got the basic version down pretty fast but the Unicode part threw me off more than I'd like to admit.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the requirements: define alphanumeric (likely Unicode letters and digits) and palindrome (case-insensitive). Then present a two-pointer solution that skips non-alphanumeric characters, analyzes time and space complexity, and discusses trade-offs with alternative approaches. Finally, outline a comprehensive test suite covering edge cases like empty strings, special characters, and Unicode.

Pro tip: Mention that Unicode case folding is more complex than simple lowercasing (e.g., 'ß' vs 'SS'), and that using a regex like [^a-zA-Z0-9] is insufficient for Unicode. This shows depth and awareness of real-world internationalization concerns.

1. Clarify requirements and assumptions

Define what 'alphanumeric' means (Unicode letters and digits) and confirm that case-insensitivity should use Unicode case folding. Ask if the input is ASCII-only or may contain Unicode, as this affects implementation.

2. Present the algorithm

Describe a two-pointer approach: initialize left and right pointers at the ends, skip non-alphanumeric characters, compare characters case-insensitively, and move pointers inward. Alternatively, mention a simpler approach that filters the string first, then checks palindrome, but note the extra space.

3. Analyze time and space complexity

State that the two-pointer approach runs in O(n) time and O(1) extra space (excluding input storage). If filtering is used, space becomes O(n). Discuss trade-offs: filtering is simpler but uses more memory.

4. Write tests covering edge cases

List test cases: empty string, single character, all non-alphanumeric, mixed case, special characters, Unicode letters (e.g., 'é'), Unicode digits, and strings with combining characters. Include a test for a palindrome with punctuation and spaces.

5. Discuss trade-offs and optimizations

Compare the two-pointer method with filtering, and mention that for very large strings, the two-pointer approach is more memory-efficient. Also note that Unicode case folding may require additional handling (e.g., using str.casefold() in Python).

Key Points to Mention

Definition of alphanumeric: use Unicode categories (e.g., isalnum() in Python) rather than ASCII-only regex.
Case-insensitivity: use case folding (e.g., str.casefold()) instead of lower() for proper Unicode handling.
Two-pointer technique: O(n) time, O(1) space, skipping non-alphanumeric characters.
Alternative approach: filter string first (O(n) space) then check palindrome; simpler but less efficient.
Edge cases: empty string, single character, all non-alphanumeric, mixed case, Unicode characters (e.g., 'é', 'ß'), combining characters.
Testing: include unit tests for each edge case, and consider property-based testing for robustness.

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