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Bytedance·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
Jun 2026

Summary

Bytedance SWE interview with a string problem that sounds straightforward until you actually try to implement it cleanly under pressure.

Questions Asked (1)

Q1

Given a string and an integer k, find the length of the longest substring where every character appears at least k times.

Algorithms & Data Structures
Author's notes

My first instinct was sliding window and I started going down that path before realizing it gets messy fast because the window validity condition isn't monotonic.

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AI HintsAI Generated

Suggested Approach

Use a divide-and-conquer strategy: recursively split the string at characters that appear fewer than k times, then return the maximum valid substring length from the resulting segments. This avoids brute-force checking and efficiently prunes invalid parts.

Pro tip: Mention that the algorithm runs in O(n) time on average because each character is processed a constant number of times across recursion levels, and discuss how to handle edge cases like k=1 or empty strings.

1. Clarify and Validate

Confirm the problem constraints: string length, character set (e.g., lowercase letters), and k's range. Ask if k can be 0 or greater than string length.

2. Outline Divide-and-Conquer

Explain that you'll recursively split the string at any character whose total frequency is less than k, because such a character cannot be part of a valid substring.

3. Implement Recursive Function

Write a function that counts character frequencies in the current substring, finds a violating character, splits at all its occurrences, and recurses on each segment.

4. Handle Base Cases

If the substring length is less than k, return 0. If no violating character exists, return the substring length.

5. Analyze Complexity and Optimize

Discuss time complexity: O(n) average due to each character being processed in one recursion level; worst-case O(n^2) if many splits. Mention potential optimizations like using a frequency map.

Key Points to Mention

  • Divide-and-conquer approach: split at characters with frequency < k.
  • Recursive implementation with base cases for length < k and no violating characters.
  • Time complexity analysis: O(n) average, O(n^2) worst-case.
  • Space complexity: O(n) due to recursion stack and frequency maps.
  • Edge cases: k=1 (whole string), k > string length (return 0), empty string.
  • Alternative approaches: sliding window with bitmask (for small alphabets) or brute-force (for comparison).

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