0200 - Number of Islands

0200 - 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.

Examples

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

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'.

Java Solution (DFS)

class Solution {
    public int numIslands(char[][] grid) {
        if(grid == null || grid.length == 0) return 0;
        
        int result = 0;
        for(int i = 0; i < grid.length; i++) {
            for(int j = 0; j < grid[0].length; j++) {
                if(grid[i][j] == '1') {
                    result++;
                    dfsHelper(grid, i, j);
                }
            }
        }
        return result;
    }
    
    private void dfsHelper(char[][] grid, int row, int column) {
        if(row < 0 || row >= grid.length || column < 0 || column >= grid[0].length || grid[row][column] == '0') return;
        
        grid[row][column] = '0';
        dfsHelper(grid, row-1, column);
        dfsHelper(grid, row+1, column);
        dfsHelper(grid, row, column-1);
        dfsHelper(grid, row, column+1);
    }
}