<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 {
private:
    void dfs(vector<vector<char>>& board, int x, int y, vector<vector<int>>& vis){
        int m = board.size(), n = board[0].size();
        
        board[x][y] = 'Y';
        vis[x][y] = 1;
        
        vector<pair<int, int>>  dirs = {{0,-1},{0,1},{-1,0},{1,0}};
        
        for(auto dir: dirs){
            int i = x + dir.first;
            int j = y + dir.second;
            
            if(i<0 || j<0 || i>m-1 || j>n-1 || board[i][j] == 'X' || board[i][j] == 'Y' || vis[i][j] == 1) continue;
            
            
            dfs(board, i, j, vis);
        }
    }
    
public:
    void solve(vector<vector<char>>& board) {
        
        int m = board.size(), n = board[0].size();
        
        vector<vector<int>> vis(m, vector<int> (n, 0));
        
        for(int i=0; i<m; i++){
            for(int j=0; j<n; j++){
                if(board[i][j] == 'O' && vis[i][j] == 0 && (i==0 || i==m-1 || j==0 || j==n-1)){
                    // mark these all connected O' with Y
                    dfs(board, i, j, vis);
                }
            }
        }
        
        for(int i=0; i<m; i++){
            for(int j=0; j<n; j++){
                if(board[i][j] == 'Y') board[i][j] = 'O';
                else if(board[i][j] == 'O') board[i][j] = 'X';
            }
        }
    }
};
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