<p>You are given an <code>m x n</code> matrix <code>board</code> containing <strong>letters</strong> <code>&#39;X&#39;</code> and <code>&#39;O&#39;</code>, <strong>capture regions</strong> that are <strong>surrounded</strong>:</p>

<ul>
	<li><strong>Connect</strong>: A cell is connected to adjacent cells horizontally or vertically.</li>
	<li><strong>Region</strong>: To form a region <strong>connect every</strong> <code>&#39;O&#39;</code> cell.</li>
	<li><strong>Surround</strong>: The region is surrounded with <code>&#39;X&#39;</code> cells if you can <strong>connect the region </strong>with <code>&#39;X&#39;</code> cells and none of the region cells are on the edge of the <code>board</code>.</li>
</ul>

<p>A <strong>surrounded region is captured</strong> by replacing all <code>&#39;O&#39;</code>s with <code>&#39;X&#39;</code>s in the input matrix <code>board</code>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">board = [[&quot;X&quot;,&quot;X&quot;,&quot;X&quot;,&quot;X&quot;],[&quot;X&quot;,&quot;O&quot;,&quot;O&quot;,&quot;X&quot;],[&quot;X&quot;,&quot;X&quot;,&quot;O&quot;,&quot;X&quot;],[&quot;X&quot;,&quot;O&quot;,&quot;X&quot;,&quot;X&quot;]]</span></p>

<p><strong>Output:</strong> <span class="example-io">[[&quot;X&quot;,&quot;X&quot;,&quot;X&quot;,&quot;X&quot;],[&quot;X&quot;,&quot;X&quot;,&quot;X&quot;,&quot;X&quot;],[&quot;X&quot;,&quot;X&quot;,&quot;X&quot;,&quot;X&quot;],[&quot;X&quot;,&quot;O&quot;,&quot;X&quot;,&quot;X&quot;]]</span></p>

<p><strong>Explanation:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2021/02/19/xogrid.jpg" style="width: 367px; height: 158px;" />
<p>In the above diagram, the bottom region is not captured because it is on the edge of the board and cannot be surrounded.</p>
</div>

<p><strong class="example">Example 2:</strong></p>

<div class="example-block">
<p><strong>Input:</strong> <span class="example-io">board = [[&quot;X&quot;]]</span></p>

<p><strong>Output:</strong> <span class="example-io">[[&quot;X&quot;]]</span></p>
</div>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>m == board.length</code></li>
	<li><code>n == board[i].length</code></li>
	<li><code>1 &lt;= m, n &lt;= 200</code></li>
	<li><code>board[i][j]</code> is <code>&#39;X&#39;</code> or <code>&#39;O&#39;</code>.</li>
</ul>

class Solution {
public:
    void solve(vector<vector<char>>& board) {
        auto n_row = board.size();
        auto n_col = board[0].size();
        //find an O
        for(int i = 1; i < n_row-1; ++i){
            for(int j = 1; j < n_col-1; ++j){
                if (board[i][j] == 'O'){
                    //start a BFS, use a map to store indices to switch
                    //map<int,vector<int>> to_capture;
                    stack<pair<int,int>> q;
                    unordered_set<int> visited;
                    q.push({i,j});
                    visited.insert(i*n_col + j);
                    bool cap = true;
                    while(!q.empty() and cap){
                        auto c = q.top();
                        q.pop();
                        //to_capture[c.first].push_back(c.second);
                        //visited.insert(c.first * c.second + c.second);
                        if(board[c.first-1][c.second] == 'O'){
                            if(c.first-1 == 0){
                                cap = false;
                            } else if(visited.find((c.first-1) * n_col + c.second) == visited.end()){
                                q.push({c.first-1, c.second});
                                visited.insert((c.first-1) * n_col + c.second);
                            }
                        }
                        if(board[c.first+1][c.second] == 'O'){
                            if(c.first+1 == n_row - 1){
                                cap = false;
                            } else if(visited.find((c.first+1) * n_col + c.second) == visited.end()){
                                q.push({c.first+1, c.second});
                                visited.insert((c.first+1) * n_col + c.second);
                            }
                        }
                        if(board[c.first][c.second-1] == 'O'){
                            if(c.second-1 == 0){
                                cap = false;
                            } else if(visited.find(c.first * n_col + c.second-1) == visited.end()){
                                q.push({c.first, c.second-1});
                                visited.insert(c.first * n_col + c.second-1);
                            }
                        }
                        if(board[c.first][c.second+1] == 'O'){
                            if(c.second+1 == n_col - 1){
                                cap = false;
                            } else if(visited.find(c.first * n_col + c.second+1) == visited.end()){
                                q.push({c.first, c.second+1});
                                visited.insert(c.first * n_col + c.second+1);
                            }
                        }
                    }
                    if(cap){
                        //std::cout << "cap ij " << i << " " << j << endl;
                        //for(const auto & m : to_capture){
                            //for(const auto & d : m.second){
                                //cout << "md " << m.first << " " << d << endl;
                                //board[m.first][d] = 'X';
                            //}
                        //}
                        for (const auto & v : visited){
                            board[v / n_col][v % n_col] = 'X';
                        }
                        //for (const auto & v : visited)
                        //    cout << "vis " << v << endl;
                    }
                }
            }
        }    

    }
};
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