Optimize compression for O(1) space
Last updated: February 24, 2026
Quick Overview
Design an algorithm to compress a given string such that it uses O(1) additional space. The input will be a string consisting of lowercase letters, and the output should be the length of the compressed string. The compression should replace consecutive repeating characters with a single character followed by the count of repetitions, but only if the compressed length is less than the original string.
Slack
February 24, 20263
12
4,899 solved
Design an algorithm to compress a given string such that it uses O(1) additional space. The input will be a string consisting of lowercase letters, and the output should be the length of the compressed string. The compression should replace consecutive repeating characters with a single character followed by the count of repetitions, but only if the compressed length is less than the original string.
This coding problem is frequently asked during Take-home Project at Slack. The interviewer is testing your ability to translate a problem into clean, working code while discussing time and space complexity. Slack expects candidates to write production-quality code, not just solve the puzzle.
What the Interviewer Expects
- Recognize the underlying problem pattern (sliding window, two pointers, BFS/DFS, etc.)
- Discuss multiple approaches and trade-offs before coding
- Implement an optimal solution with clean, production-quality code
- Handle all edge cases including boundary conditions and invalid input
- Optimize both time and space complexity with clear justification
- Test your solution systematically with well-chosen examples
Key Topics to Cover
How to Approach This
- Clarify input constraints and edge cases before writing code.
- Walk through your approach verbally and confirm with the interviewer before coding.
- Start with a brute force solution, then optimize. Mention time and space complexity.
- Test your solution with examples, including edge cases like empty input or duplicates.
- Consider common patterns: sliding window, two pointers, hash map, BFS/DFS, dynamic programming.
Possible Follow-up Questions
- How would you parallelize this solution?
- What happens if the input contains duplicates?
- How would your solution change if the input was sorted?
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Practice DSA ProblemsSample Answer
Problem Analysis
The problem requires us to compress a given string by replacing consecutive repeating characters with a single character followed by the count of repetitions, while ensuring that the compressed length...
Approach
- Initialize two pointers:
i(for reading the string) andj(for writing the compressed version). - Loop through the string using
ito identify sequences of the same character. - For each...
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