Optimize partitioning for O(n) time
Last updated: June 10, 2026
Quick Overview
Given an array of integers, partition the array into two subsets such that all elements in one subset are less than or equal to a given pivot, and all elements in the other subset are greater than the pivot. Implement this partitioning in O(n) time complexity, ensuring that the relative order of elements is maintained, and return the modified array as output.
Doordash
June 10, 20262
15
2,319 solved
Given an array of integers, partition the array into two subsets such that all elements in one subset are less than or equal to a given pivot, and all elements in the other subset are greater than the pivot. Implement this partitioning in O(n) time complexity, ensuring that the relative order of elements is maintained, and return the modified array as output.
Coding interviews at Doordash focus on problem-solving approach as much as the final solution. The interviewer wants to see you break down the problem, consider edge cases, and optimize iteratively. Communication throughout the process is key.
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 your solution change if the input was sorted?
- Can you solve this iteratively instead of recursively (or vice versa)?
- How would you modify your solution to handle streaming input?
- How would you test this solution thoroughly?
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Problem Analysis
To solve this problem, we can utilize the two pointers technique. This approach is ideal here because we need to traverse the array only once (O(n) time complexity) while maintaining the relative ...
Approach
- Initialize two pointers:
leftstarting from the beginning of the array andrightstarting from the end of the array. - Create a result array to store the partitioned output.
- Traverse the ar...
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