Transform matrix to adjacency list

Last updated: September 28, 2025

Quick Overview

Convert the given array to sorted result following the specified rules.

Reddit
Coding & Algorithms
Machine Learning Engineer
Reddit
September 28, 2025
Machine Learning Engineer
Phone Screen
Coding & Algorithms
Medium

568

4

622 solved


Convert the given array to sorted result following the specified rules.

Reddit uses this problem in the Phone 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
Sorting and searching
Binary search and divide and conquer
Dynamic programming and memoization
Time and space complexity analysis
Hash maps and frequency counting
Common algorithm patterns (sliding window, two pointers, BFS/DFS)
How to Approach This
  1. Clarify input constraints and edge cases before writing code.
  2. Walk through your approach verbally and confirm with the interviewer before coding.
  3. Start with a brute force solution, then optimize. Mention time and space complexity.
  4. Test your solution with examples, including edge cases like empty input or duplicates.
  5. 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)?
  • What is the worst-case input for your solution?
  • How would you modify your solution to handle streaming input?
  • How would you parallelize this solution?
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Sample Answer
Problem Analysis

Start by understanding the input/output format and constraints. Identify the core problem pattern. Ask clarifying questions: Is the input sorted? Can ...

Brute Force Approach

Always start with the simplest correct solution, even if it's not optimal. This shows you can solve the problem and gives you a baseline to optimize f...


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