Optimize flattening for O(1) space
Last updated: December 3, 2025
Quick Overview
Given a nested list of integers, implement a function to flatten the list into a single list of integers while optimizing for O(1) space complexity. The input will be a list that may contain integers or other nested lists, and the output should be a single flat list containing all the integers in the same order they appear.
Square/Block
December 3, 20258
4
161 solved
Given a nested list of integers, implement a function to flatten the list into a single list of integers while optimizing for O(1) space complexity. The input will be a list that may contain integers or other nested lists, and the output should be a single flat list containing all the integers in the same order they appear.
This coding problem is frequently asked during Onsite at Square/Block. The interviewer is testing your ability to translate a problem into clean, working code while discussing time and space complexity. Square/Block 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
- Can you solve this iteratively instead of recursively (or vice versa)?
- Can you solve this in a single pass?
- How would you parallelize this solution?
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Practice DSA ProblemsSample Answer
Problem Analysis
The problem requires flattening a nested list of integers while maintaining O(1) space complexity. This suggests that we should not use additional data structures that grow with the input size, such a...
Approach
- Start with the given nested list and initialize an index to track our position.
- Iterate through the list. If the current element is an integer, we can simply place it in a position of the list ...
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