BFS on grid
Last updated: November 23, 2025
Quick Overview
Given a 2D grid of 0s and 1s, where 0 represents an empty cell and 1 represents an obstacle, implement a Breadth-First Search (BFS) algorithm to find the shortest path from the top-left corner to the bottom-right corner of the grid. Return the length of the path, or -1 if no path exists.
MongoDB
November 23, 202525
11
4,809 solved
Given a 2D grid of 0s and 1s, where 0 represents an empty cell and 1 represents an obstacle, implement a Breadth-First Search (BFS) algorithm to find the shortest path from the top-left corner to the bottom-right corner of the grid. Return the length of the path, or -1 if no path exists.
MongoDB 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
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
- What if the input doesn't fit in memory?
- How would you parallelize this solution?
- What is the worst-case input for your solution?
- How would your solution change if the input was sorted?
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Practice DSA ProblemsSample Answer
Problem Analysis
This problem can be efficiently solved using the Breadth-First Search (BFS) algorithm because it explores all possible paths from the starting point (top-left corner) level by level, ensuring that we ...
Approach
- Initialization: Start by checking if the starting point (0,0) or the destination (n-1,m-1) is an obstacle (1). If either is blocked, return -1 immediately.
- Queue setup: Use a queue to fa...
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