200. Number of Islands

Problem Link

Description

---

Given an m x n 2D binary grid grid which represents a map of '1's (land) and '0's (water), return 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: grid = [
  ["1","1","1","1","0"],
  ["1","1","0","1","0"],
  ["1","1","0","0","0"],
  ["0","0","0","0","0"]
]
Output: 1

Example 2:

Input: grid = [
  ["1","1","0","0","0"],
  ["1","1","0","0","0"],
  ["0","0","1","0","0"],
  ["0","0","0","1","1"]
]
Output: 3

 

Constraints:

• m == grid.length
• n == grid[i].length
• 1 <= m, n <= 300
• grid[i][j] is '0' or '1'.

Solution

---

Python3

class Solution:
    def numIslands(self, grid: List[List[str]]) -> int:
        rows, cols = len(grid), len(grid[0])
        res = 0
 
        def go(x, y):
            grid[x][y] = "0"
 
            for dx, dy in [(x + 1, y), (x - 1, y), (x, y + 1), (x, y - 1)]:
                if 0 <= dx < rows and 0 <= dy < cols and grid[dx][dy] == "1":
                    go(dx, dy)
        
        for i in range(rows):
            for j in range(cols):
                if grid[i][j] == "1":
                    go(i, j)
                    res += 1
 
        return res