130. Surrounded Regions**

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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 XX O O XX X O XX O X X

After running your function, the board should be:

X X X XX X X XX X X XX 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);            }}

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