## Description

https://leetcode.com/problems/number-of-islands/

Given a 2d grid map of `'1'`

s (land) and `'0'`

s (water), count 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.

**Example 1:**

Input:11110 11010 11000 00000Output:1

**Example 2:**

Input:11000 11000 00100 00011Output:3

## Explanation

if encounter ‘1’, an island found, use dfs to visit all its lands, and mark to “0”. Once the process finished. The next encountered ‘1’ suggests another island found.

## Python Solution

```
class Solution:
def numIslands(self, grid: List[List[str]]) -> int:
count = 0
for i in range(0, len(grid)):
for j in range(0, len(grid[0])):
if grid[i][j] == '1':
self.island_helper(grid, i, j)
count += 1
return count
def island_helper(self, grid, i, j):
if i < 0 or i > len(grid) - 1:
return
if j < 0 or j > len(grid[0]) -1:
return
if grid[i][j] == '0':
return
grid[i][j] = '0'
self.island_helper(grid, i + 1, j)
self.island_helper(grid, i - 1, j)
self.island_helper(grid, i, j + 1)
self.island_helper(grid, i, j - 1)
```

- Time complexity: O(N).
- Space complexity: O(N).

## 2 Thoughts to “LeetCode 200. Number of Islands”