Number of Islands
Last updated: June 17, 2026
Quick Overview
Given a 2D grid of '1's (land) and '0's (water), count the number of islands. Tests BFS/DFS on grids, one of Google's most frequently asked problems.
Google
Coding & Algorithms
Software Engineer
Software Engineer
Phone Screen
Coding & Algorithms
Medium
384
0
54 solved
Given a 2D grid of '1's (land) and '0's (water), count the number of islands. Tests BFS/DFS on grids, one of Google's most frequently asked problems.
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.
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Problem Analysis
The problem of counting the number of islands in a 2D grid is best solved using Depth-First Search (DFS) or Breadth-First Search (BFS). Here, the grid is composed of '1's representing land and '0's re...
Approach
- Initialize a counter to track the number of islands.
- Iterate through each cell in the 2D grid.
- When a '1' is found, increment the island counter.
- Perform a DFS or BFS from that cell to vis...
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