BFS on grid
Last updated: May 24, 2026
Quick Overview
Given a 2D grid of 0s and 1s, where 0 represents water and 1 represents land, implement a Breadth-First Search (BFS) algorithm to find the number of connected components of land in the grid. Your function should take the grid as input and return an integer representing the count of distinct islands.
Lyft
May 24, 20269
5
1,195 solved
Given a 2D grid of 0s and 1s, where 0 represents water and 1 represents land, implement a Breadth-First Search (BFS) algorithm to find the number of connected components of land in the grid. Your function should take the grid as input and return an integer representing the count of distinct islands.
Coding interviews at Lyft 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 your solution change if the input was sorted?
- What happens if the input contains duplicates?
- How would you modify your solution to handle streaming input?
- What is the worst-case input for your solution?
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Problem Analysis
The problem requires us to find the number of distinct islands in a 2D grid where '1' represents land and '0' represents water. This suggests using the Breadth-First Search (BFS) algorithm, which is w...
Approach
- Initialize a counter for the number of islands.
- Create a visited grid to keep track of which cells have been processed.
- Loop through each cell in the grid:
- If a cell contains '1' and has...
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