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()
answer = test.numIslands([
    ['1', '1', '1', '1', '0'],
    ['1', '1', '0', '1', '0'],
    ['1', '1', '0', '0', '0'],
    ['0', '0', '0', '0', '0']
])
assert answer == 1
answer = test.numIslands([])
assert answer == 0
answer = test.numIslands([
    ['1', '1', '0', '0', '0'],
    ['1', '1', '0', '0', '0'],
    ['0', '0', '1', '0', '0'],
    ['0', '0', '0', '1', '1']
])
assert answer == 3
print('All Passed!')

Big O Analysis

Space Complexity: O(1)

Time Complexity: O(N), N = number of cells