Number of Islands Python

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

Using DFS Method:

class Solution:
    def numIslands(self, grid: List[List[str]]) -> int:
        if not grid:
            return 0
        rows = len(grid)
        cols = len(grid[0])
        def dfs(i,j,grid):
            if i < 0 or i >= rows or j >= cols or j < 0 or grid[i][j] == '0':
                return 
            grid[i][j] = "0"
            dfs(i+1,j,grid)
            dfs(i-1,j,grid)
            dfs(i,j+1,grid)
            dfs(i,j-1,grid)
            return 1
        noOfIslands = 0
        for i in range(0,rows):
            for j in range(0,cols):
                if grid[i][j] == "1":
                    noOfIslands+=dfs(i,j,grid)
        return noOfIslands
        
        

Time Complexity : O(M * N)

Space Complexity : O(M * N) Worst case