Number of Islands

Question

## Problem

Given an `m × n` binary grid where `"1"` is land and `"0"` is water, return the **number of islands**.

An island is surrounded by water and formed by connecting adjacent land cells horizontally or vertically.

## Example
```
grid = [
  ["1","1","0","0"],
  ["1","1","0","0"],
  ["0","0","1","0"],
  ["0","0","0","1"]
]
Output: 3
```

## Follow-ups Google commonly asks
1. What if the grid is too large to fit in memory — it streams row by row?
2. Solve using Union-Find instead of DFS/BFS
3. Count islands where diagonal connections also count