<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<pair<int,int>> dirs{{0,1},{0,-1},{1,0},{-1,0}};
    void solve(vector<vector<char>>& board) {
        for(int i = 0; i < board.size(); i++)
        {
            for(int j = 0; j < board[0].size(); j++)
            {
                if(i == 0 || j == 0 || i == board.size()-1 || j == board[0].size()-1 )
                {
                    if(board[i][j] == 'O')
                    {
                        dfs(i,j,board,'E');
                    }
                }
            }
        }
        for(int i = 0; i < board.size(); i++)
        {
            for(int j = 0; j < board[0].size(); j++)
            {
                if(board[i][j] == 'O')
                {
                    dfs(i,j,board,'X');
                }
            }
        }
        for(int i = 0; i < board.size(); i++)
        {
            for(int j = 0; j < board[0].size(); j++)
            {
                if(board[i][j] == 'E')
                {
                    board[i][j] = 'O';
                }
            }
        }
    }
    void dfs(int row, int col, vector<vector<char>>& board, char fill)
    {
        board[row][col] = fill;
        for(auto dir:dirs)
        {
            int nrow = row  + dir.first;
            int ncol = col  + dir.second;
            if(nrow >= 0 && ncol >= 0 && nrow < board.size() && ncol < board[0].size() && board[nrow][ncol] == 'O')
            {
                dfs(nrow,ncol,board,fill);
            }
        }
    }
};
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