Number of Islands

Easy
Code
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Variables

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Output

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Solution Guide: Number of Islands

Approach

Visit every cell. When land is found, run DFS to mark every connected land cell as water. Increment a counter only for a new starting land component.

Complexity Analysis

Time
O(m * n)
Space
O(m * n)

Pattern

Graph as Grid Components

Why It Works

Every island is a connected region. Once DFS marks all cells in one island as visited, remaining unvisited land must belong to a new island.

Updated Mar 2026