DFS

class Solution {
   public static void f(char[][] grid,boolean[][] isVisited,int i,int j)
    {
        if(i<0||i>=grid.length||j<0||j>=grid[0].length||grid[i][j]=='0'||isVisited[i][j]) return ;
        isVisited[i][j]=true;
        f(grid,isVisited,i+1,j);
        f(grid,isVisited,i,j+1);
        f(grid,isVisited,i-1,j);
        f(grid,isVisited,i,j-1);
    }
    public  static int numIslands(char[][] grid)
    {
        if(grid==null||grid.length==0) return 0;
        boolean[][] isVisited=new boolean[grid.length][grid[0].length];
        int count=0;
        for(int i=0;i<grid.length;i++)
        {
            for(int j=0;j<grid[0].length;j++)
            {
                if(grid[i][j]=='1'&&!isVisited[i][j])
                {
                    f(grid,isVisited,i,j);
                    count++;
                }
            }
        }
        return count;
    }
}

BFS

 public static class Pos{
        int i;
        int j;
        public Pos(int i,int j){
            this.i=i;
            this.j=j;
        }
    }

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

        int count=0;
        boolean[][] isVisited=new boolean[grid.length][grid[0].length];
        LinkedList<Pos> queue=new LinkedList<>();
        for(int i=0;i<grid.length;i++)
        {
            for(int j=0;j<grid[0].length;j++)
            {
                if(!isVisited[i][j]&&grid[i][j]=='1')
                {
                    count++;
                    queue=new LinkedList<>();
                    queue.addLast(new Pos(i,j));
                    isVisited[i][j]=true;
                    while(!queue.isEmpty())
                    {
                        Pos position=queue.pollFirst();
                        int curi=position.i;
                        int curj=position.j;
                        //四个方向看是否合法，然后加入到队列中来
                        //向上
                        if(curi!=0&&!isVisited[curi-1][curj]&&grid[curi-1][curj]=='1')
                        {
                            queue.addLast(new Pos(curi-1,curj));
                            isVisited[curi-1][curj]=true;
                        }
                        //向下
                        if(curi!=grid.length-1&&!isVisited[curi+1][curj]&&grid[curi+1][curj]=='1')
                        {
                            queue.addLast(new Pos(curi+1,curj));
                            isVisited[curi+1][curj]=true;
                        }
                        //向左
                        if(curj!=0&&!isVisited[curi][curj-1]&&grid[curi][curj-1]=='1')
                        {
                            queue.addLast(new Pos(curi,curj-1));
                            isVisited[curi][curj-1]=true;
                        }
                        //向右
                        if(curj!=grid[0].length-1&&!isVisited[curi][curj+1]&&grid[curi][curj+1]=='1')
                        {
                            queue.addLast(new Pos(curi,curj+1));
                            isVisited[curi][curj+1]=true;
                        }
                    }
                }
            }
        }
        return count;
    }