<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();
        vector<vector<int>> vis(m, vector<int>(n, 0));
        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                if (board[i][j] == 'O')
                    board[i][j] = 'S';
            }
        }

        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                if (board[i][j] == 'S' && !vis[i][j] &&
                    (i == 0 || i == m - 1 || j == 0 || j == n - 1)) {
                    // cout << "i: " << i << " j: " << j << endl;
                    pair<int, int> na = {i, j};
                    queue<pair<int, int>> qu;
                    qu.push(na);
                    vis[i][j] = 1;
                    while (!qu.empty()) {
                        auto node = qu.front();
                        qu.pop();
                        int p = node.first;
                        int q = node.second;
                        board[p][q] = 'O';
                        if (q + 1 < n && board[p][q + 1] == 'S' &&
                            !vis[p][q + 1]) {
                            qu.push({p, q + 1});
                            vis[p][q + 1] = 1;
                        }
                        if (p + 1 < m && board[p + 1][q] == 'S' &&
                            !vis[p + 1][q]) {
                            qu.push({p + 1, q});
                            vis[p + 1][q] = 1;
                            // board[p][q+1]='X';
                        }
                        if (q - 1 >= 0 && board[p][q - 1] == 'S' &&
                            !vis[p][q - 1]) {
                            qu.push({p, q - 1});
                            vis[p][q - 1] = 1;
                            // board[p][q+1]='X';
                        }
                        if (p - 1 >= 0 && board[p - 1][q] == 'S' &&
                            !vis[p - 1][q]) {
                            qu.push({p - 1, q});
                            vis[p - 1][q] = 1;
                            // board[p][q+1]='X';
                        }
                    }
                }
            }
        }
         for (int i = 0; i < m; i++) {
                for (int j = 0; j < n; j++) {
                    if (board[i][j] == 'S')
                        board[i][j] = 'X';
                }
         }
    }
};
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