<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:
    bool valid(vector<vector<char>>& board,vector<vector<int>> &vis,int i,int j ,int n,int m)
    {
        return (i<n && i>=0 && j<m && j>=0 && vis[i][j]==-1 && board[i][j]=='O');
    }
    bool dfs(vector<vector<char>>& board,vector<vector<int>> &vis,int i,int j ,int n,int m,vector<pair<int,int>> &current){
        if(i==0 || i==n-1 || j==0 || j==m-1){

            return false;
        }
        current.push_back({i,j});
        vis[i][j]=1;
        bool a,b,c,d;
        a=true;
        b=true;
        c=true;
        d=true;
        if(valid(board,vis,i+1,j,n,m)){
            a=dfs(board,vis,i+1,j,n,m,current);
        }
        if(valid(board,vis,i-1,j,n,m)){
            b=dfs(board,vis,i-1,j,n,m,current);
        }
        if(valid(board,vis,i,j-1,n,m)){
            c=dfs(board,vis,i,j-1,n,m,current);
        }
        if(valid(board,vis,i,j+1,n,m)){
            d=dfs(board,vis,i,j+1,n,m,current);
        }
        return a&&b&&c&&d;
    }
    void solve(vector<vector<char>>& board) {
        int n = board.size();
        int m = board[0].size();
        vector<vector<int>> vis(n,vector<int>(m,-1));
        for(int i=1;i<n-1;i++){
            for(int j=1;j<m-1;j++){
                if(board[i][j]=='O' && vis[i][j]==-1){
                    vector<pair<int,int>> current;
                    bool a=dfs(board,vis,i,j,n,m,current);
                    
                    if(a) {
                        int s=current.size();
                        for(int k=0;k<s;k++){
                            board[current[k].first][current[k].second]='X';
                        }
                    }
                }
            }
        }

    }
};
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