130. Surrounded Regions**

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本文介绍了一种解决二维棋盘中被'X'包围的'O'区域问题的算法。通过深度优先搜索（DFS）从边界'O'开始标记，避免内部被包围的'O'被错误替换为'X'。最终实现将所有未被标记的'O'替换为'X'，而保留边界相连的'O'不变。

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Given a 2D board containing 'X' and 'O' (the letter O), capture all regions surrounded by 'X'.

A region is captured by flipping all 'O's into 'X's in that surrounded region.

For example,

X X X X
X O O X
X X O X
X O X X

After running your function, the board should be:

X X X X
X X X X
X X X X
X O X X

dfs：

public class Solution {
    private int m;
    private int n;
    public void solve(char[][] board) {
        n=board.length;
        if(n==0) return;
        m=board[0].length;
        for(int i=0;i<n;i++){
            if(board[i][0]=='O') dfs(board,i,0);
            if(board[i][m-1]=='O') dfs(board,i,m-1);
        }
        for(int j=0;j<m;j++){
            if(board[0][j]=='O') dfs(board,0,j);
            if(board[n-1][j]=='O') dfs(board,n-1,j);
        }
        
        for(int i=0;i<n;i++){
            for(int j=0;j<m;j++){
                if(board[i][j]=='O') board[i][j]='X';
                if(board[i][j]=='*') board[i][j]='O';
            }
        }
    }
    private void dfs(char[][] board,int i, int j){
        if(i<0||j<0||i>n-1||j>m-1) return;
        if(board[i][j]=='O')
            board[i][j]='*';
        if(i>1 &&board[i-1][j]=='O') dfs(board,i-1,j);
        if(i<n-2&&board[i+1][j]=='O') dfs(board,i+1,j);
        if(j>1&&board[i][j-1]=='O') dfs(board,i,j-1);
        if(j<m-2&&board[i][j+1]=='O') dfs(board,i,j+1);
        

    }
}