<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 {
private:
    void bfs(vector<vector<char>>& board, vector<vector<int>>& vis, int row,
             int col) {
        queue<pair<int, int>> q;
        q.push({row, col});
        vis[row][col] = 1;

        int rows[] = {0, -1, 0, 1};
        int cols[] = {-1, 0, 1, 0};

        int m = board.size();
        int n = board[0].size();

        while (!q.empty()) {
            int r = q.front().first;
            int c = q.front().second;
            q.pop();

            for (int k = 0; k < 4; k++) {
                int n_r = r + rows[k];
                int n_c = c + cols[k];

                if (n_r < m && n_r >= 0 && n_c < n && n_c >= 0 &&
                    !vis[n_r][n_c] && board[n_r][n_c] == 'O') {
                    q.push({n_r, n_c});
                    vis[n_r][n_c] = 1;
                }
            }
        }
    }

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 < n; i++) {
            if (board[0][i] == 'O') {
                bfs(board, vis, 0, i);
            }
            if (board[m - 1][i] == 'O') {
                bfs(board, vis, m - 1, i);
            }
        }

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

        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                if (board[i][j] == 'O' && !vis[i][j]) {
                    board[i][j] = 'X';
                }
            }
        }
    }
};
16073