Bytedance·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Jun 2026

Summary

Bytedance software engineering interview with a string manipulation problem that looks straightforward until you realize brute force blows up on nested removals. The stack insight is clean once you see it, but getting there under pressure is a different story.

Questions Asked (1)

Given a string, repeatedly remove every substring matching the pattern lowercase-uppercase-lowercase (where the first and third characters are the same letter), scanning left to right, until no more such substrings remain. Return the final string.

Algorithms & Data Structures

Author's notes

My first instinct was to just loop and replace until nothing changes, which works but felt gross.

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AI HintsAI Generated

Suggested Approach

Clarify the problem constraints and edge cases, then propose a stack-based solution that processes the string in one pass, simulating the removal of patterns. Explain how the stack efficiently handles nested and cascading removals by checking the top three characters after each push.

Pro tip: Mention that a naive repeated scan is O(n^2) and that the stack approach achieves O(n) time and space, which is crucial for large inputs. Also, discuss how to handle the pattern when the third character matches the first, ensuring you check the correct indices.

1. Understand the problem

Restate the problem in your own words and confirm the pattern: lowercase-uppercase-lowercase with first and third characters identical. Ask about constraints (e.g., string length, character set) and edge cases (empty string, no matches).

2. Discuss naive approach and its limitations

Explain that repeatedly scanning the string and removing matches is straightforward but inefficient (O(n^2) time). Highlight that this may not scale for large inputs.

3. Propose optimized stack-based solution

Describe using a stack to process characters one by one. After pushing each character, check if the top three form the pattern; if so, pop them. This simulates the removal and handles cascading effects in O(n) time.

4. Walk through an example

Choose a small example (e.g., 'aAbBcC' or 'abBA') and demonstrate step-by-step how the stack processes it, showing when removals occur and the final result.

5. Analyze complexity and edge cases

State time and space complexity (O(n) each). Discuss edge cases: empty string, no matches, all matches, and strings with multiple overlapping patterns.

Key Points to Mention

Pattern definition: lowercase, uppercase, lowercase with first and third characters equal.
Naive approach: repeated scanning and removal leads to O(n^2) time.
Stack-based solution: process characters, check top three for pattern, pop if matched.
Time and space complexity: O(n) time, O(n) space.
Handling cascading removals: stack naturally handles nested and overlapping patterns.
Edge cases: empty string, no matches, all characters removed, and large inputs.

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