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Number of Islands

Problem Statement​

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.

Leetcode link

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'.

Code​

Python Code
class Solution:
    def numIslands(self, grid: List[List[str]]) -> int:
        if not grid: return 0
        row = len(grid)
        col = len(grid[0])
        visited = set()
        count = 0
        directions=[(-1,0),(0,1),(1,0),(0,-1)]
        def findIsland(x,y):
            for dx, dy in directions:
                nx,ny = x+dx, y+dy
                if 0<=nx<row and 0<=ny<col and grid[nx][ny]=='1' and (nx,ny) not in visited:
                    visited.add((nx,ny))
                    findIsland(nx,ny)

        for x in range(row):
            for y in range(col):
                if grid[x][y] == '1' and (x,y) not in visited:
                    count +=1
                    visited.add((x,y))
                    findIsland(x,y)
        return count