<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 {
    bool seen;
    void mark(vector<vector<char>>& board,int i,int j,int r,int c)
    {
        if(i<0 || i>r-1 || j<0 || j>c-1)
            return;
        if(board[i][j]=='X')
            return;
        
        board[i][j] = 'X';
        mark(board,i-1,j,r,c);
        mark(board,i+1,j,r,c);
        mark(board,i,j-1,r,c);
        mark(board,i,j+1,r,c);       
    }
    void dfs(vector<vector<char>>& board,int i,int j,int r,int c,vector<vector<bool>>& visited)
    {
        if(i<0 || i>r-1 || j<0 || j>c-1)
            return;
        
        if(board[i][j]=='X' || visited[i][j])
            return;
        if(i<=0 || i>=r-1 || j<=0 || j>=c-1)
            seen = true;
        
        visited[i][j] = true;
        dfs(board,i-1,j,r,c,visited);
        dfs(board,i+1,j,r,c,visited);
        dfs(board,i,j-1,r,c,visited);
        dfs(board,i,j+1,r,c,visited);
    }
public:
    void solve(vector<vector<char>>& board) {
        ios_base::sync_with_stdio(false);
        cin.tie(NULL);
        
        int r=board.size();
        if(r<=1)
            return;
        int c=board[0].size();
        if(c<=1)
            return;
        vector<vector<bool>> visited(r,vector<bool>(c,false));
        
        for(int i=1;i<r-1;++i)
        {
            for(int j=1;j<c-1;++j)
            {
                if(board[i][j]=='O' && !visited[i][j])
                {
                    seen = false;
                    dfs(board,i,j,r,c,visited);
                    if(seen==false)
                        mark(board,i,j,r,c);
                    seen = true;
                }
            }
        }
    }
};

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