Optimize flattening for in-place
Last updated: July 26, 2025
Quick Overview
Given a nested list of integers, implement an in-place algorithm to flatten the list into a single-level list while maintaining the order of elements. The input will be a list that can contain integers and other lists, and the output should be a modified version of the input list that contains only integers in a flat structure. Ensure that the solution uses constant space and does not create any additional data structures for the flattening process.
Datadog
July 26, 20259
3
371 solved
Given a nested list of integers, implement an in-place algorithm to flatten the list into a single-level list while maintaining the order of elements. The input will be a list that can contain integers and other lists, and the output should be a modified version of the input list that contains only integers in a flat structure. Ensure that the solution uses constant space and does not create any additional data structures for the flattening process.
Datadog 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
- 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
- What happens if the input contains duplicates?
- How would your solution change if the input was sorted?
- How would you test this solution thoroughly?
- Can you solve this iteratively instead of recursively (or vice versa)?
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Problem Analysis
The problem requires us to flatten a nested list of integers in-place, which means we cannot use any extra data structures that grow with input size. The key pattern here is Depth-First Search (DFS) s...
Approach
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Initialize a pointer: We will use an index pointer to track our position in the list.
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Iterate through the list: For each element, check whether it is an integer or a list.
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**Handl...
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