<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:
    int dx[4] = {0, 0, 1, -1};
    int dy[4] = {-1, 1, 0, 0};
    void solve(vector<vector<char>>& board) {
        int m = board.size();
        int n = board[0].size();

        map <pair<int,int>, int> mp;
        vector <vector<int>> vis(m, vector <int> (n, 0));
        queue <pair<int, int>> q;
        for(int i = 0; i < n; i++){
            if(board[0][i] == 'O'){
                q.push({0, i});
            }
        }
        for(int i = 0; i < m; i++){
            if(board[i][n-1] == 'O'){
                q.push({i, n-1});
            }
        }

        for(int i = 0; i < m ; i++){
            if(board[i][0] == 'O'){
                q.push({i, 0});
            }
        }

        for(int i = 0; i < n; i++){
            if(board[m-1][i] == 'O'){
                q.push({m-1, i});
            }
        }


        while(!q.empty()){
            auto it = q.front();
            q.pop();

            int x = it.first;
            int y = it.second;

            mp[{x, y}] = 1;

            if(vis[x][y])continue;
            else vis[x][y] = 1;


            for(int i = 0; i < 4; i++){
                int nx = x + dx[i];
                int ny = y + dy[i];

                if(nx >= 0 && nx < m && ny >= 0 && ny < n && board[nx][ny] == 'O'){
                    q.push({nx, ny});
                }
            }
        }


        for(int i = 0; i < m; i ++){
            for(int j = 0; j < n;j++){
                if(mp[{i, j}] == 0){
                    board[i][j] = 'X';
                }
            }
        }
    }
};
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