<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 {
    void dfs(int i, int j, vector<vector<char>>& board, vector<vector<int>>& visited, bool& flag){
        if(i < 0 || j < 0 || i >= board.size() || j >= board[0].size() || board[i][j] == 'X' || visited[i][j]){
            return;
        }
        if(i == 0 || j == 0 || i == board.size() - 1 || j == board[0].size() - 1){
            flag = false;
        }
        visited[i][j] = 1;
        dfs(i + 1, j, board, visited, flag);        
        dfs(i, j + 1, board, visited, flag);        
        dfs(i - 1, j, board, visited, flag);        
        dfs(i, j - 1, board, visited, flag);        
    }
    
    void change(int i, int j, vector<vector<char>>& board){
        if(i < 0 || j < 0 || i == board.size() || j == board[0].size() || board[i][j] == 'X'){
            return;
        }
        board[i][j] = 'X';
        change(i+1, j, board);
        change(i, j+1, board);
        change(i-1, j, board);
        change(i, j-1, board);

    }
    
public:
    void solve(vector<vector<char>>& board) {
        int n = board.size(), m = board[0].size();
        for(int i=0;i<n;i++){
            for(int j=0;j<m;j++){
                if(board[i][j] == 'O' && i != 0 && j != 0 && i != n-1 && j != m-1){
                    vector<vector<int>> visited(n,vector<int>(m,0));
                    bool f = true;
                    dfs(i, j, board, visited, f);
                    if(f) change(i, j, board);
                }
            }
        }
    }
};
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