Number of Islands

Difficulty: Easy · Topic: Pathfinding

Given a grid of 0 (water) and 1 (land), return the number of islands. An island is a group of 1s connected 4-directionally (up/down/left/right). Flood-fill each unvisited land cell with BFS or DFS and count how many fills you start.

Examples

num_islands([[1,1,0,0],[1,0,0,1],[0,0,1,1]])
→ 2

A left L-shape and a right block.

num_islands([[1]])
→ 1

Target complexity: time O(R·C), space O(R·C)

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