//duynotes.blogspot.com
class Solution {
    public int numIslands(char[][] grid) {
        if (grid==null || grid.length == 0 || grid[0].length==0){
            return 0;
        }
        int m = grid.length;
        int n = grid[0].length;
        int count =0;
        
        for (int i=0; i<m; i++){
            for (int j=0; j<n; j++){
                if (grid[i][j]=='1'){
                    count++;
                    DFS(grid,i,j);
                }
            }
        }
        return count;
    }
    
    private void DFS(char[][] grid, int i, int j){
        int m = grid.length;
        int n = grid[0].length;
        
        if (i<0 || i>=m || j<0 || j>=n || grid[i][j]=='0'){
            return;
        }
        
        grid[i][j]='0';
        
        DFS(grid,i-1,j);
        DFS(grid,i+1,j);
        DFS(grid,i,j-1);
        DFS(grid,i,j+1);
    }
}