<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 {
    vector<vector<int>> vis; 
public:
    void solve(vector<vector<char>>& grid) {
        int n = grid.size(); 
        int m = grid[0].size(); 

        vis.assign(n, vector<int>(m, 0)); 
        for(int i=0; i<n; i++){
            for(int j=0; j<m; j++){
                if(grid[i][j] == 'O' && vis[i][j] == 0){
                    vis.assign(n, vector<int>(m, 0));
                    dfs(i, j, grid);

                    bool works = true;
                    for(int p=0; p<n; p++){
                        works = works && (vis[p][0] == 0);
                        works = works && (vis[p][m-1] == 0);
                    }
                    for(int p=0; p<m; p++){
                        works = works && (vis[0][p] == 0);
                        works = works && (vis[n-1][p] == 0);
                    }

                    if(works){
                        for(int p=0; p<n; p++){
                            for(int q=0; q<m; q++){
                                cout << vis[p][q] << "  ";
                                if(vis[p][q]){
                                    grid[p][q] = 'X';
                                }
                            }
                            cout << endl;
                        }
                        cout << "============================\n";
                    }
                }
            }
        }

    }

    void dfs(int i, int j, vector<vector<char>>& grid){
        vis[i][j] = 1;
        int n = grid.size();
        int m = grid[0].size();

        assert(grid[i][j] == 'O'); 

        if(i>0 && !vis[i-1][j] && grid[i-1][j] == 'O'){
            dfs(i-1, j, grid); 
        }

        if(i<n-1 && !vis[i+1][j] && grid[i+1][j] == 'O'){
            dfs(i+1, j, grid); 
        }

        if(j>0 && !vis[i][j-1] && grid[i][j-1] == 'O'){
            dfs(i, j-1, grid); 
        }

        if(j<m-1 && !vis[i][j+1] && grid[i][j+1] == 'O'){
            dfs(i, j+1, grid); 
        }
    }
};
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