<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:
    void solve(vector<vector<char>>& board) {
        int m=board.size();
        int n=board[0].size();
        queue<pair<int,int>>q;
        vector<vector<bool>>vis(m,vector<bool>(n,0));
        for(int j=0; j<n; j++){
            if(board[0][j]=='O'){
                q.push({0,j});
                vis[0][j]=1;
            }
        }
        for(int i=0; i<m; i++){
            if(board[i][0]=='O'){
                q.push({i,0});
                vis[i][0]=1;
            }
        }
        for(int i=1; i<m; i++){
            if(board[i][n-1]=='O'){
                q.push({i,n-1});
                vis[i][n-1]=1;
            }
        }
        for(int j=n-2; j>0; j--){
            if(board[m-1][j]=='O'){
                q.push({m-1,j});
                vis[m-1][j]=1;
            }
        }
        int delrow[4]={0,-1,0,1};
        int delcol[4]={1,0,-1,0};
        while(!q.empty()){
            int row=q.front().first;
            int col=q.front().second;
            q.pop();
            for(int i=0; i<4; i++){
                int nrow=row+delrow[i];
                int ncol=col+delcol[i];
                if(nrow>=0 && ncol>=0 && nrow<m && ncol<n && board[nrow][ncol]=='O' && !vis[nrow][ncol]){
                    vis[nrow][ncol]=1;
                    q.push({nrow,ncol});
                }
            }
        }
        for(int i=0; i<m; i++){
            for(int j=0; j<n; j++){
                if(vis[i][j]==0 && board[i][j]=='O'){
                    board[i][j]='X';
                }
            }
        }
    }
};
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