Optimize flattening for O(n) time
Last updated: October 2, 2025
Quick Overview
Given a nested list of integers, write a function to flatten the list into a single list of integers in O(n) time, where n is the total number of integers in the nested structure. The function should handle varying levels of nesting and return a new list containing all the integers in the same order they appear.
Elastic
October 2, 202582
11
4,575 solved
Given a nested list of integers, write a function to flatten the list into a single list of integers in O(n) time, where n is the total number of integers in the nested structure. The function should handle varying levels of nesting and return a new list containing all the integers in the same order they appear.
This coding problem is frequently asked during Technical Screen at Elastic. The interviewer is testing your ability to translate a problem into clean, working code while discussing time and space complexity. Elastic expects candidates to write production-quality code, not just solve the puzzle.
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
- Can you optimize the space complexity of your solution?
- What happens if the input contains duplicates?
- What if the input doesn't fit in memory?
- Can you solve this iteratively instead of recursively (or vice versa)?
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Problem Analysis
This problem requires us to flatten a nested list of integers into a single list while preserving the order of elements. The optimal approach here is a Depth-First Search (DFS) traversal pattern, whic...
Approach
- Initialize a result list: Start with an empty list that will hold all the integers.
- Define a recursive function: Create a helper function that takes a sublist as input. For each elemen...
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