Java is too slow to pass the large test.

Surrounded Regions.java

public class Solution {
    public void solve(char[][] board) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if( board==null  || board.length==0 || board[0].length==0) return; 
        int m=board.length;
        int n=board[0].length;
        for(int i=0; i<m; i++)
        {
            if(board[i][0]=='O')   DFS(board, i, 0);
            if(board[i][n-1]=='O') DFS(board, i, n-1);
        }
        for(int j=1; j<n-1; j++)
        {
            if(board[0][j]=='O')   DFS(board, 0, j);
            if(board[m-1][j]=='O') DFS(board, m-1, j);
        }
        for(int i=0; i<m; i++)
        {
            for(int j=0; j<n; j++)
            {
                if(board[i][j]=='O') board[i][j]='X';
                if(board[i][j]=='D') board[i][j]='O';
            }
        }
    }
    void DFS(char [][] board, int i, int j)
    {
        board[i][j]='D';
        if(i+1<board.length && board[i+1][j]=='O') DFS(board, i+1, j);
        if(i-1>=0 && board[i-1][j]=='O') DFS(board, i-1, j);
        if(j+1<board[0].length && board[i][j+1]=='O') DFS(board, i, j+1);
        if(j-1>=0 && board[i][j-1]=='O') DFS(board, i, j-1);
    }
}