<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, map<pair<int,int>, bool>& vis, int i, int j)
    {
        int r = board.size(), c = board[0].size();
        if (i < 0 || j < 0 || i>= r || j >= c || vis[{i,j}])
        {
            return;
        }

        vis[{i,j}] = true;

        if (i+1 < r && board[i+1][j] == 'O')
            dfs(board, vis, i+1, j);
        if (i-1 >= 0 && board[i-1][j] == 'O')
            dfs(board, vis, i-1, j);
        if (j+1 < c && board[i][j+1] == 'O')
            dfs(board, vis, i, j+1);
        if (j-1 >= 0 && board[i][j-1] == 'O')
            dfs(board, vis, i, j-1);
    }

    void solve(vector<vector<char>>& board) {
        int r = board.size(), c = board[0].size();
        map<pair<int,int>, bool> vis;
        for (int i=0; i<r; i++)
        {
            if (vis.find({i,0}) == vis.end()&& board[i][0] == 'O')
            {
                dfs(board, vis, i, 0);
            }

            if (vis.find({i,c-1}) == vis.end() && board[i][c-1] == 'O')
            {
                dfs(board, vis, i, c-1);
            }
        }

        for (int i=0; i<c; i++)
        {
            if (vis.find({0,i}) == vis.end() && board[0][i] == 'O')
            {
                dfs(board, vis, 0, i);
            }

            if (vis.find({r-1,i}) == vis.end() && board[r-1][i] == 'O')
            {
                dfs(board, vis, r-1, i);
            }
        }

        for (int i=0; i<r; i++){
            for (int j=0; j<c; j++)
            {
                // cout<<vis[{i,j}]<<", ";
                if (!vis[{i,j}])
                {
                    board[i][j] = 'X';
                }
            }
            // cout<<"\n";
        }
    }
};
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