Optimize partitioning for in-place
Last updated: May 14, 2026
Quick Overview
Given an array of integers, implement an in-place algorithm to optimize the partitioning of the array such that all elements less than a specified pivot are moved to the left, and all elements greater than or equal to the pivot are moved to the right. The function should modify the array in place and return the final partitioned array. The expected time complexity is O(n), and the space complexity should be O(1).
Two Sigma
May 14, 20269
7
3,549 solved
Given an array of integers, implement an in-place algorithm to optimize the partitioning of the array such that all elements less than a specified pivot are moved to the left, and all elements greater than or equal to the pivot are moved to the right. The function should modify the array in place and return the final partitioned array. The expected time complexity is O(n), and the space complexity should be O(1).
Two Sigma uses this problem in the Technical Screen to evaluate your algorithmic thinking. They expect you to discuss multiple approaches, analyze trade-offs between them, and implement the optimal solution with clean, readable code.
What the Interviewer Expects
- Quickly identify the optimal approach and its theoretical basis
- Handle complex algorithm design with multiple interacting components
- Write concise, elegant code under time pressure
- Prove correctness of your approach and discuss alternative solutions
- Optimize beyond the obvious: discuss constant factor improvements
- Address follow-up variations and explain how the solution generalizes
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
- What happens if the input contains duplicates?
- Can you solve this in a single pass?
- How would you parallelize this solution?
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Problem Analysis
To solve this problem, we can leverage the two pointers technique. The main goal is to rearrange the array in such a way that all elements less than the pivot are to the left and all elements grea...
Approach
- Initialize two pointers:
leftstarting from the beginning of the array (index 0) andrightstarting from the end of the array (indexlen(array)-1). - Iterate through the array using ...
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