<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) {
        vector<vector<char>> visited(board.size(), vector<char>(board[0].size(), 0));
        for (int i = 0; i < board.size(); ++i)
            for (int j = 0; j < board[i].size(); ++j)
                if (!visited[i][j] && board[i][j] == 'O') {
                    bool touch = false;
                    dfs(i, j, touch, board, visited);
                    if (!touch)
                        dfs2(i, j, touch, board, visited);
                }
    }

    void dfs(int i, int j, bool& touch, vector<vector<char>>& board, vector<vector<char>>& visited) {
        if (visited[i][j])
            return;
        visited[i][j] = 1;
        if (i == 0 || j == 0 || i + 1 == visited.size() || j + 1 == visited[i].size())
            touch = true;

        if (i != 0 && board[i - 1][j] == 'O')
            dfs(i - 1, j, touch, board, visited);
        if (i + 1 < visited.size() && board[i + 1][j] == 'O')
            dfs(i + 1, j, touch, board, visited);
        if (j != 0 && board[i][j - 1] == 'O')
            dfs(i, j - 1, touch, board, visited);
        if (j + 1 < visited[i].size() && board[i][j + 1] == 'O')
            dfs(i, j + 1, touch, board, visited);
    }

    void dfs2(int i, int j, bool& touch, vector<vector<char>>& board, vector<vector<char>>& visited) {
        if (visited[i][j] == 2)
            return;
        visited[i][j] = 2;
        board[i][j] = 'X';

        if (i != 0 && board[i - 1][j] == 'O')
            dfs2(i - 1, j, touch, board, visited);
        if (i + 1 < visited.size() && board[i + 1][j] == 'O')
            dfs2(i + 1, j, touch, board, visited);
        if (j != 0 && board[i][j - 1] == 'O')
            dfs2(i, j - 1, touch, board, visited);
        if (j + 1 < visited[i].size() && board[i][j + 1] == 'O')
            dfs2(i, j + 1, touch, board, visited);
    }
};
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