<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) {
        if (board.empty())
              return;

                  constexpr int dirs[4][2] = {{0, 1}, {1, 0}, {0, -1}, {-1, 0}};
                      const int m = board.size();
                          const int n = board[0].size();

                              queue<pair<int, int>> q;

                                  for (int i = 0; i < m; ++i)
                                        for (int j = 0; j < n; ++j)
                                                if (i * j == 0 || i == m - 1 || j == n - 1)
                                                          if (board[i][j] == 'O') {
                                                                      q.emplace(i, j);
                                                                                  board[i][j] = '*';
                                                                                            }

                                                                                                // Mark the grids that stretch from the four sides with '*'.
                                                                                                    while (!q.empty()) {
                                                                                                          const auto [i, j] = q.front();
                                                                                                                q.pop();
                                                                                                                      for (const auto& [dx, dy] : dirs) {
                                                                                                                              const int x = i + dx;
                                                                                                                                      const int y = j + dy;
                                                                                                                                              if (x < 0 || x == m || y < 0 || y == n)
                                                                                                                                                        continue;
                                                                                                                                                                if (board[x][y] != 'O')
                                                                                                                                                                          continue;
                                                                                                                                                                                  q.emplace(x, y);
                                                                                                                                                                                          board[x][y] = '*';
                                                                                                                                                                                                }
                                                                                                                                                                                                    }

                                                                                                                                                                                                        for (vector<char>& row : board)
                                                                                                                                                                                                              for (char& c : row)
                                                                                                                                                                                                                      if (c == '*')
                                                                                                                                                                                                                                c = 'O';
                                                                                                                                                                                                                                        else if (c == 'O')
                                                                                                                                                                                                                                                  c = 'X';
    }
};
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