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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 :

Input:
11000
11000
00100
00011

Output: 3

Solution:
We will solve this problem using DFS(Depth First Search) Algorithm tracing all the 1’s until we find no 1’s on 4 sides recursively then we will increase the count.

class Solution {
    public int numIslands(char[][] grid) {
        if(grid.length==0) return 0;
        int rows = grid.length;
        int columns = grid[0].length;
        int noOfIslands = 0;
        for(int i=0;i<rows;i++){
            for(int j=0;j<columns;j++){
                if(grid[i][j]=='1'){
                    dfs(grid, i, j);
                    noOfIslands++;
                }
            }
        }
        
        return noOfIslands;
    }
    
    private void dfs(char[][] grid, int i, int j){
        if(i<0 || j<0 ||i>=grid.length|| j>=grid[0].length|| grid[i][j]!='1') return;
        grid[i][j] = '2';
        dfs(grid, i+1, j);
        dfs(grid, i, j+1);
        dfs(grid, i-1, j);
        dfs(grid, i, j-1);
        
    }
}