200. Number of Islands

Posted 2020-09-26 Premiumlab

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

 

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:

11110
11010
11000
00000

Answer: 1

Example 2:

11000
11000
00100
00011

Answer: 3

 

 

Sol:

 

class Solution(object):
    def numIslands(self, grid):
        """
        :type grid: List[List[str]]
        :rtype: int
        """
        
        # DFS
        # Iterate through each of the cell and if it is an island, do dfs to mark all adjacent islands, then increase the counter by 1.
        if not grid:
            return 0
        
        res = 0
        for i in range(len(grid)):
            for j in range(len(grid[0])):
                if grid[i][j] == ‘1‘:
                    res += 1
                    self.dfs(grid, i, j)
                
        return res
    
    def dfs(self, grid, i, j):
        if i < 0 or j < 0 or i >= len(grid) or j >= len(grid[0]) or grid[i][j] != ‘1‘:
            return 
        
        grid[i][j] = ‘#‘
        self.dfs(grid, i+1, j)
        self.dfs(grid, i-1, j)
        self.dfs(grid, i, j+1)
        self.dfs(grid, i, j-1)
        

 

 

Note:

 

1 Matrix  grid is represented by array list, then use grid[i][j] to represent each element.