<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:

    vector<vector<int>> adjacent_nodes(int i, int j, int m, int n){
        vector<vector<int>> ans;
        
        if (i > 0) ans.push_back({i - 1, j}); 
        if (j > 0) ans.push_back({i, j - 1}); 
        if (i < m - 1) ans.push_back({i + 1, j});
        if (j < n - 1) ans.push_back({i, j + 1});
    
        return ans;
    }

    void DFS(vector<vector<char>>& board, vector<vector<bool>>& visited, int i, int j, int m, int n){
        visited[i][j] = true;

        vector<vector<int>> adj = adjacent_nodes(i, j, m, n);

        for (auto v:adj){
            if (board[v[0]][v[1]] == 'O'){
                if (!visited[v[0]][v[1]]) DFS(board, visited, v[0], v[1], m, n);
            }
        }
    }

    void solve(vector<vector<char>>& board) {
        int m = board.size();
        int n = board[0].size();

        vector<vector<bool>> visited(m, vector<bool>(n, false));
        
        for (int i = 0; i < m; i++){
            if ((board[i][0] == 'O') && (!visited[i][0])) DFS(board, visited, i, 0, m, n);
            if ((board[i][n - 1] == 'O') && (!visited[i][n - 1])) DFS(board, visited, i, n - 1, m, n);
        }

        for (int j = 0; j < n; j++){
            if ((board[0][j] == 'O') && (!visited[0][j])) DFS(board, visited, 0, j, m, n);
            if ((board[m - 1][j] == 'O') && (!visited[m - 1][j])) DFS(board, visited, m - 1, j, m, n);
        }

        for (int i = 0; i < m; i++){
            for(int j = 0; j < n; j++){
                if (board[i][j] == 'O' && !visited[i][j]) board[i][j] = 'X';
            }
        }

    }
};
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