floodfill算法系列一＞岛屿数量

class Solution {
    int m,n,ret;
    boolean[][] vis;

    public int numIslands(char[][] grid) {
        m = grid.length;
        n = grid[0].length;

        vis = new boolean[m][n];
        for(int i = 0; i < m; i++)
            for(int j = 0; j < n; j++){
                if(!vis[i][j] && grid[i][j] == '1'){
                    ret++;
                    //把这个岛屿附近的1标记一下
                    dfs(grid,i,j);
                }
            }

        return ret;    
    }

    int[] dx = {0,0,-1,1};
    int[] dy = {1,-1,0,0};
    private void dfs(char[][] grid, int i, int j){

        vis[i][j] = true;//标记
        for(int k = 0; k < 4; k++){
            int x = i + dx[k]; int y = j + dy[k];
            if(x >= 0 && x < m && y >= 0 && y < n && !vis[x][y] && grid[x][y] == '1'){
                dfs(grid,x,y);
            }
        }

    }
}