1 void replace(vector<vector<char> > &board, int i, int j, char before, char after) { 2   stack<pair<int, int> > st; 3   int rlen = board.size(); 4   int clen = board[0].size(); 5   int rnext[4] = {-1, 1, 0, 0}; 6   int cnext[4] = {0, 0, -1, 1}; 7  8   st.push(pair<int, int>(i, j)); 9   while (!st.empty()) {10     pair<int, int> top = st.top();11     st.pop();12     if (top.first < 0 || top.first >= rlen || top.second < 0 || top.second >= clen || board[top.first][top.second] != before)13       continue;14     board[top.first][top.second] = after;15     for (int k = 0; k < 4; k++)16       st.push(pair<int, int>(top.first + rnext[k], top.second + cnext[k]));17   }18 }19 20 void solve(vector<vector<char>> &board) {21   if (board.empty() || board[0].empty()) return;22   int rlen = board.size();23   int clen = board[0].size();24 25   for (int i = 0; i < rlen; i++) {26     replace(board, i, 0, ‘O‘, ‘#‘);27     replace(board, i, clen - 1, ‘O‘, ‘#‘);28   }29   for (int j = 0; j < clen; j++) {30     replace(board, 0, j, ‘O‘, ‘#‘);31     replace(board, rlen - 1, j, ‘O‘, ‘#‘);32   }33 34   for (int i = 1; i < rlen - 1; i++)35     for (int j = 1; j < clen - 1; j++)36       if (board[i][j] == ‘O‘)37         board[i][j] = ‘X‘;38 39   for (int i = 0; i < rlen; i++) {40     replace(board, i, 0, ‘#‘, ‘O‘);41     replace(board, i, clen - 1, ‘#‘, ‘O‘);42   }43   for (int j = 0; j < clen; j++) {44     replace(board, 0, j, ‘#‘, ‘O‘);45     replace(board, rlen - 1, j, ‘#‘, ‘O‘);46   }47 }