Minimum Time for Contamination to Spread

1. Problem Statement

You are given a grid where each cell is empty (0), clean (1), or contaminated (2). Every minute, contamination spreads to horizontally or vertically adjacent clean cells. Return the minimum number of minutes for every clean cell to become contaminated, or -1 if some clean cell can never be reached.

2. Live Coding Format

Work in the provided coding workspace while explaining your decisions to the live interviewer. Validation may use inline tests, examples, comments, or a verbal walkthrough.

3. Key Focus Areas

1
Problem clarification and contract
2
Approach exploration and optimality
3
Think-aloud communication and pacing
4
Implementation correctness
5
Code quality and language fluency
6
Tests and dry run
7
Debugging and self-correction
8
Complexity analysis
9
Follow-up performance

4. What Strong Candidates Should Demonstrate

Clarify cell states, spread direction rules, and impossible-case behavior before coding.
Explain a round-by-round full-grid rescan baseline and improve it to a linear-time solution.
Think aloud while implementing readable, correct code.
Validate with explicit expected outputs and a top-to-bottom dry run.
Analyze time and space complexity and handle a bounded follow-up modification.

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Practice this problem in the live coding workspace while the interviewer probes your clarification, implementation, trade-offs, and validation.

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Core Concepts

Hidden because they name the technique, which a real interviewer would not tell you.

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Grid TraversalMulti-Source BFSQueuesComplexity Analysis

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