130.cpp

class Solution {
public:
    void solve(vector<vector<char>>& board) {
        if(board.empty() || board[0].empty())   return;
        
        int rows = board.size();
        int cols = board[0].size();
        
        for(int i = 0;i < rows;i++){
            if(board[i][0] == 'O')  dfs(board,i,0);
            if(board[i][cols - 1] == 'O') dfs(board,i,cols - 1);
        }
        
        for(int j = 0;j < cols;j++){
            if(board[0][j] == 'O')  dfs(board,0,j);
            if(board[rows - 1][j] == 'O') dfs(board,rows - 1,j);
        }
        
        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] == '#') board[i][j] = 'O';
            }
        }
    }
    
    void dfs(vector<vector<char>> &board,int row,int col){
        if(row < 0 || col < 0 || row >= board.size() || col >= board[row].size() || board[row][col] != 'O') return;
        
        board[row][col] = '#';
        
        dfs(board,row - 1,col) , dfs(board,row + 1,col) , dfs(board,row,col - 1) , dfs(board,row,col + 1);
    }
};