Optimize flattening for streaming input
Last updated: April 18, 2026
Quick Overview
Given a stream of nested lists, implement a function that optimizes the flattening of these lists into a single flat list. The function should handle streaming input efficiently, ensuring that it can process elements as they arrive without storing the entire input in memory. The output should be a flat list containing all the elements from the nested lists in the order they were received.
MongoDB
April 18, 202639
7
4,562 solved
Given a stream of nested lists, implement a function that optimizes the flattening of these lists into a single flat list. The function should handle streaming input efficiently, ensuring that it can process elements as they arrive without storing the entire input in memory. The output should be a flat list containing all the elements from the nested lists in the order they were received.
Coding interviews at MongoDB 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 you parallelize this solution?
- What happens if the input contains duplicates?
- 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 nested lists in a streaming fashion, which hints at the use of a recursive depth-first search (DFS) approach. Each element can either be an integer or another nested...
Approach
- Initialize an empty list to store the flattened elements.
- Define a recursive function that accepts a nested list:
- If the current element is an integer, append it to the output l...
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