Number of Islands

 

Number of Islands

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

Solution

class Solution {

public void dfs(char[][] grid, int i, int j) {

int nr = grid.length;

int nc = grid[0].length;

if(i<0 || i>=nr || j<0 || j>=nc || grid[i][j]=='0'){

return;

}

grid[i][j]='0';

// check for any connected islands in all 4 directions

dfs(grid, i-1, j); // top

dfs(grid, i+1, j); // bottom

dfs(grid, i, j-1); // left

dfs(grid, i, j+1); // right

}

public int numIslands(char[][] grid) {

int nr = grid.length;

int nc = grid[0].length;

int islandsCount = 0;

for(int i=0; i<nr; i++) {

for(int j=0; j<nc; j++) {

if(grid[i][j]=='1'){

islandsCount++;

dfs(grid, i, j);

}

}

}

return islandsCount;

}

}