Number of Islands

Question

## Problem

Count islands in an m×n binary grid (4-directional).

## Example
```
[[1,1,0],[0,1,0],[0,0,1]] -> 2
```

## Constraints
- 1 ≤ m,n ≤ 300

## Approach
DFS/BFS or union-find. Maps to grouping connected objects on a board.