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
Google
June 17, 2026
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
  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.
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