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class Solution {
int rows, cols;
public:
    void solve(vector<vector<char>>& board) {
        rows = board.size();
        cols = board[0].size();

        for(int i=0; i<rows; i++)
            for(int j=0; j<cols; j++)
                if(i*j == 0 || i == rows-1 || j == cols-1)
                    dfs(i, j, board, '1');

        for(int i=0; i<rows; i++)
            for(int j=0; j<cols; j++)
                if(board[i][j] == 'O')
                    board[i][j] = 'X';
                else if(board[i][j] == '1')
                    board[i][j] = 'O';
    }

    void dfs(int x, int y, vector<vector<char>>& board, char c){
        if(x < 0 || x >= rows || y < 0 || y >= cols || board[x][y] != 'O') return;
        board[x][y] = c;
        dfs(x+1, y, board, c);
        dfs(x, y+1, board, c);
        dfs(x-1, y, board, c);
        dfs(x, y-1, board, c);
    }
};