<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) {
      int n = board.size();
        if (n == 0) return;
        int m = board[0].size();

        for (int i = 0; i < n; ++i) 
            for (int j = 0; j < m; ++j) 
            {
            if(board[i][j]=='O') 
            DFS(board,i,j);
            } 
    }
    void DFS(vector<vector<char>>& board,int i,int j)
    {
        stack<pair<int, int>> s;
        stack<pair<int, int>> check_surround;
        s.push({i, j});
        bool needrecover=false;
        while(!s.empty())
        {
            pair<int,int> current=s.top();
            s.pop();
            int x = current.first;
            int y = current.second;

            if(board[x][y]=='O')
            {
            board[x][y] = 'X'; // Example: mark as visited.
            check_surround.push(current);
            }
            else
            continue;

            // Ensure the indices are within bounds and the cell hasn't been visited.
            if (x == 0 || x == board.size()-1 || y == 0 || y == board[0].size()-1) {
                needrecover=true;
                break;
            }
            
            // Push adjacent cells onto the stack (up, down, left, right).
            s.push({x + 1, y});
            s.push({x - 1, y});
            s.push({x, y + 1});
            s.push({x, y - 1});
        }

        if(needrecover)
        {
            while(!check_surround.empty())
            {
                pair<int,int> current=check_surround.top();
                check_surround.pop(); 
                int x = current.first;
                int y = current.second;

                board[x][y] = 'O'; // Example: mark as novisited.
            }
        }
        
    }
};
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