What is the Number of Islands pattern?

The Number of Islands is a common coding interview pattern in Depth First Search problems. It provides a structured approach to solving related algorithm challenges efficiently.

When should I use Number of Islands?

Use the Number of Islands pattern when you encounter problems matching its characteristics. Check the "When to Use" and "Clues" sections in the lesson for specific indicators.

Is Number of Islands asked in FAANG interviews?

Yes, Number of Islands is an essential pattern commonly tested in FAANG and top tech company technical interviews.

Browse Curriculum

Depth-First Search

Medium

Number of Islands

Count the number of connected groups of 1s in a grid.

10 min read

Solve on LeetCode

Examples

Input:grid = [['1','1','0'], ['1','1','0'], ['0','0','1']]

Output:2

Explain:

Island 1 (Top Left), Island 2 (Bottom Right).

Problem Understanding

Given an m x n map of '1's (land) and '0's (water), return the number of islands.
An island is surrounded by water and formed by connecting adjacent lands horizontally or vertically.

Algorithm Strategy

We need to find Connected Components.

Iterate: Loop through every cell (r, c) of the grid.
Trigger: If we find a land cell '1', we found a NEW island!
- Increment count.
- Sink the Island: Launch DFS from this cell to visit all connected lands and mark them as visited (e.g., change '1' to '0').
Continue: If we see a '0' (or visited cell), skip it.

Visualization

Step 1 / 1

Visualization data missing

Initializing...

Custom Input

Watch as we iterate through the grid. When we hit land, invalidating the entire connected chunk.

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