> ## Documentation Index
> Fetch the complete documentation index at: https://leetcode-py.wisl.dev/llms.txt
> Use this file to discover all available pages before exploring further.

> ## Agent Instructions
> leetcode-py is a Python LeetCode practice environment generator with one CLI: lcpy. It is not a service or platform.
> Each problem is a directory under leetcode/ with README.md, solution.py, test_solution.py, helpers.py, and playground.ipynb. lcpy gen creates them from JSON templates bundled with the package.
> Examples are backed by tests; copy them verbatim.

# Number of Islands Python Solution with Tests

> Tested Python solution for LeetCode 200 with 14 pytest cases. Generate a practice environment with lcpy.

LeetCode 200, Medium. Topics: Array, Depth-First Search, Breadth-First Search, Union Find, Matrix. [View on LeetCode](https://leetcode.com/problems/number-of-islands/description/).

Generate this problem as a practice environment: tested reference solution, 14 [parametrized pytest cases](/practice/testing), and a playground notebook:

```bash theme={"theme":{"light":"github-light","dark":"github-dark"}}
lcpy gen -n 200   # by problem number
lcpy gen -s number_of_islands   # by problem name
```

## Problem

Given an `m x n` 2D binary grid `grid` which represents a map of `'1'`s (land) and `'0'`s (water), return *the number of islands*.

An **island** is surrounded by water and is formed by connecting adjacent lands horizontally or vertically. You may assume all four edges of the grid are all surrounded by water.

### Examples

```
Input: grid = [
  ["1","1","1","1","0"],
  ["1","1","0","1","0"],
  ["1","1","0","0","0"],
  ["0","0","0","0","0"]
]
Output: 1
```

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

### Constraints

* `m == grid.length`
* `n == grid[i].length`
* `1 <= m, n <= 300`
* `grid[i][j]` is `'0'` or `'1'`.

## Solution

Reference implementation from [solution.py on GitHub](https://github.com/wislertt/leetcode-py/blob/main/leetcode/number_of_islands/solution.py), full suite in [test\_solution.py](https://github.com/wislertt/leetcode-py/blob/main/leetcode/number_of_islands/test_solution.py):

```python theme={"theme":{"light":"github-light","dark":"github-dark"}}
class Solution:
    # Time: O(m * n) where m = rows, n = cols
    # Space: O(m * n) for recursion stack in worst case
    def num_islands(self, grid: list[list[str]]) -> int:
        if not grid or not grid[0]:
            return 0
        VISITED = "0"  # noqa: N806
        UNVISITED_ISLAND = "1"  # noqa: N806

        rows, cols = len(grid), len(grid[0])
        islands = 0

        def dfs(r: int, c: int) -> None:
            if r < 0 or r >= rows or c < 0 or c >= cols or grid[r][c] != UNVISITED_ISLAND:
                return

            grid[r][c] = VISITED
            for dr, dc in [(1, 0), (-1, 0), (0, 1), (0, -1)]:
                dfs(r + dr, c + dc)

        for r in range(rows):
            for c in range(cols):
                if grid[r][c] == UNVISITED_ISLAND:
                    islands += 1
                    dfs(r, c)

        return islands
```

## Complexity

| Time                               | Space                                       |
| ---------------------------------- | ------------------------------------------- |
| O(m \* n) where m = rows, n = cols | O(m \* n) for recursion stack in worst case |

## Tags

[Grind 75](/catalog/grind-75), [Grind](/catalog/grind), [Blind 75](/catalog/blind-75), [NeetCode 150](/catalog/neetcode-150), [NeetCode 250](/catalog/neetcode-250), [NeetCode All](/catalog/neetcode), [AlgoMaster 75](/catalog/algo-master-75).