<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 dfs(vector<vector<char>>& board, int line, int col, bool& isOnEdge, vector<int>& visited, vector<pair<int,int>>& region)
    {
        int m = board.size();
        int n = board[0].size();
        if (line < 0 || line >= m || col < 0 || col >= n || board[line][col] == 'X')
            return;
        
        if (visited[line * n + col] == 1)
            return;

        visited[line * n + col] = 1;
        if (line == 0 || col == 0 || line == m - 1 || col == n - 1)
        {
            isOnEdge = true;
        }

        dfs(board, line + 1, col, isOnEdge, visited, region);
        dfs(board, line, col + 1, isOnEdge, visited, region);
        dfs(board, line - 1, col, isOnEdge, visited, region);
        dfs(board, line, col - 1, isOnEdge, visited, region);

        region.push_back(make_pair(line, col));
    }
    void solve(vector<vector<char>>& board) {
        int m = board.size();
        int n = board[0].size();
        bool isOnEdge = false;
        vector<int> visited(m*n, 0);
    
        for (int i = 0; i < m; ++i)
        {
            for (int j = 0; j < n; ++j)
            {
                vector<pair<int,int>> region;
                if (board[i][j] == 'O' && visited[i * n + j] == 0)
                {
                    dfs(board, i, j, isOnEdge, visited, region);
                    if (!isOnEdge)
                    {
                        for (int k = 0; k < region.size(); ++k)
                        {
                            board[region[k].first][region[k].second] = 'X';
                        }
                    }
                }
                isOnEdge = false;
            }
        }
    }
};
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