<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 {
  constexpr static int dirs[][2] = {{0, 1}, {0, -1}, {1, 0}, {-1, 0}};
  vector<vector<bool>> discarded;

  void discard_region(vector<vector<char>> &board, int row, int col) {
    if (board[row][col] == 'X' || discarded[row][col])
      return;
    else {
      discarded[row][col] = true;

      for (auto [rd, cd] : dirs)
        if (!(row + rd < 0 || row + rd >= board.size() || col + cd < 0 ||
              col + cd >= board[0].size()))
          discard_region(board, row + rd, col + cd);
    }
  }

  void capture_region(vector<vector<char>> &board, int row, int col) {
    if (board[row][col] == 'X' || discarded[row][col])
      return;
    else {
      board[row][col] = 'X';

      for (auto [rd, cd] : dirs)
        if (!(row + rd < 0 || row + rd >= board.size() || col + cd < 0 ||
              col + cd >= board[0].size()))
          capture_region(board, row + rd, col + cd);
    }
  }

public:
  void solve(vector<vector<char>> &board) {
    const int rows = board.size(), cols = board[0].size();
    this->discarded = vector<vector<bool>>(rows, vector<bool>(cols));

    for (int row = 0; row < rows; ++row) {
      if (board[row][0] == 'O' && !discarded[row][0])
        discard_region(board, row, 0);
      if (board[row][cols - 1] == 'O' && !discarded[row][cols - 1])
        discard_region(board, row, cols - 1);
    }

    for (int col = 0; col < cols; ++col) {
      if (board[0][col] == 'O' && !discarded[0][col])
        discard_region(board, 0, col);
      if (board[rows - 1][col] == 'O' && !discarded[rows - 1][col])
        discard_region(board, rows - 1, col);
    }

    for (int row = 1; row < rows - 1; ++row)
      for (int col = 1; col < cols - 1; ++col)
        if (board[row][col] == 'O' && !discarded[row][col])
          capture_region(board, row, col);
  }
};
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