https://leetcode.cn/problems/surrounded-regions/
class Solution {int n;int m;boolean[][] vis;char[][] board;int[] dx=new int[]{0,1,0,-1};int[] dy=new int[]{1,0,-1,0};public void solve(char[][] board) {// 从边缘O出发,通过flood fill,标记和边缘相连的联通块,再把剩余区域标记为Xn=board.length;m=board[0].length;vis=new boolean[n][m];this.board=board;for(int i=0;i<n;i++)for(int j=0;j<m;j++){// 边缘的点满足性质: x==0||n-1 或者 y==0||m-1if(i!=0 && i!=n-1 && j!=0 && j!=m-1)continue;if(board[i][j]=='O' && vis[i][j]==false){dfs(i,j);}}for(int i=0;i<n;i++)for(int j=0;j<m;j++){if(vis[i][j]==false && board[i][j]=='O')board[i][j]='X';}} void dfs(int x,int y){vis[x][y]=true;for(int i=0;i<4;i++){int a=x+dx[i],b=y+dy[i];if(a>=0 && b>=0 && a<n && b<m && vis[a][b]==false && board[a][b]=='O'){dfs(a,b);}}}}
