Optimize flattening for O(n) time
Last updated: July 7, 2025
Quick Overview
Given a nested list of integers, implement a function that flattens the list into a single list of integers in O(n) time complexity. The input will be a list that may contain integers or other nested lists, and the output should be a single list containing all the integers in the same order they appear in the original nested structure.
Citadel
July 7, 20258
14
2,333 solved
Given a nested list of integers, implement a function that flattens the list into a single list of integers in O(n) time complexity. The input will be a list that may contain integers or other nested lists, and the output should be a single list containing all the integers in the same order they appear in the original nested structure.
This coding problem is frequently asked during Onsite at Citadel. The interviewer is testing your ability to translate a problem into clean, working code while discussing time and space complexity. Citadel 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
- How would you parallelize this solution?
- Can you optimize the space complexity of your solution?
- How would your solution change if the input was sorted?
- Can you solve this iteratively instead of recursively (or vice versa)?
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Practice DSA ProblemsSample Answer
Problem Analysis
To flatten a nested list of integers into a single list, we can recognize that the problem can be approached using a depth-first search (DFS) pattern. This is because we need to explore each element i...
Approach
- Initialize an empty list,
result, to hold the flattened integers. - Create a helper function
flattenthat takes a nested structure as input. - For each element in the input:
- If the eleme...
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