Optimize partitioning for O(n) time
Last updated: February 10, 2026
Quick Overview
Given an array of integers, optimize the partitioning of the array such that all elements less than a specified pivot are on the left and all elements greater than or equal to the pivot are on the right, achieving this in O(n) time complexity. The function should take the array and the pivot as inputs and return the rearranged array.
PlanetScale
February 10, 202610
7
864 solved
Given an array of integers, optimize the partitioning of the array such that all elements less than a specified pivot are on the left and all elements greater than or equal to the pivot are on the right, achieving this in O(n) time complexity. The function should take the array and the pivot as inputs and return the rearranged array.
This coding problem is frequently asked during Phone Screen at PlanetScale. The interviewer is testing your ability to translate a problem into clean, working code while discussing time and space complexity. PlanetScale 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
- Can you solve this iteratively instead of recursively (or vice versa)?
- How would you parallelize this solution?
- What happens if the input contains duplicates?
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Practice DSA ProblemsSample Answer
Problem Analysis
The problem requires rearranging an array such that all elements less than a specified pivot are on the left, and all elements greater than or equal to the pivot are on the right. This can be efficien...
Approach
- Initialize two pointers,
leftstarting at the beginning of the array andrightat the end. - Use a while loop to iterate until
leftis less than or equal toright. - Increment
leftunti...
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