<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) {
        int rows = board.size();
        int cols = board[0].size();
        if (rows == 0) return;
        vector<vector<int>> visited(rows, vector<int>(cols, 0));
        bool surrounded = true;
        // vector<pair<char, char>> region;
        for (int r = 0; r < rows; r++)
        {
            for (int c = 0; c < cols; c++)
            {
                if (board[r][c]== 'O' && visited[r][c] == 0)
                {
                    // region.clear();
                    vector<pair<int, int>> region;
                    surrounded = bfs(r,c,board,visited, region, rows, cols);
                    if (surrounded)
                    {
                        for (int i = 0; i < region.size(); i++)
                        {
                            board[region[i].first][region[i].second] = 'X';
                        }
                    }
                    surrounded = true;;
                }
            }
        }
    }

    bool bfs(int r, int c,vector<vector<char>>& board, vector<vector<int>> & visited, vector<pair<int, int>> & region, int rows, int cols)
    {
        if (r < 0 || r > rows - 1|| c < 0 | c > cols-1 || board[r][c]=='X' ||
            visited[r][c] == 1 ) return true;

        bool result = true;
        visited[r][c] = 1;
    
        bool result1 = bfs(int(r-1), c, board, visited, region, rows, cols);
        bool result2 = bfs(int(r+1), c, board, visited, region, rows, cols); 
        bool result3 = bfs(r, int(c-1), board, visited, region, rows, cols); 
        bool result4 = bfs(r, int(c+1), board, visited, region, rows, cols);
        if (!(result1 && result2 && result3 && result4)) result = false;
        if (r == 0 || r == rows-1 || c == 0|| c == cols-1) result = false;
        region.push_back(make_pair(r,c));
        return result;
    }
};
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