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
00000


Output: 1

Example 2:

Input:

11000
11000
00100
00011


Output: 3

## Solution

from typing import List

class Solution:
def numIslands(self, grid: List[List[str]]) -> int:
if not grid:
return 0

numIslands = 0
for i in range(len(grid)):
for j in range(len(grid[0])):
if grid[i][j] == '1':
self.dfs(grid, i, j)
numIslands += 1

return numIslands

def dfs(self, grid: List[List[str]], row, col) -> None:
if row < 0 or row >= len(grid) or col < 0 or col >= len(grid[0]):
return

if grid[row][col] != '1':
return

grid[row][col] = '2'
self.dfs(grid, row-1, col)
self.dfs(grid, row+1, col)
self.dfs(grid, row, col-1)
self.dfs(grid, row, col+1)


## Test Cases

test = Solution()
['1', '1', '1', '1', '0'],
['1', '1', '0', '1', '0'],
['1', '1', '0', '0', '0'],
['0', '0', '0', '0', '0']
])
['1', '1', '0', '0', '0'],
['1', '1', '0', '0', '0'],
['0', '0', '1', '0', '0'],
['0', '0', '0', '1', '1']
])