<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:
    unordered_set<int> s; unordered_map<int, int> v;

    void bfs(int st, vector<vector<char>>& board) {
        int n = board.size(); int m = board[0].size();
        bool b = true; queue<int> q; q.push(st); s.insert(st); v[st] = 1;
        while(!q.empty()) {
            int o = q.front(); q.pop(); int i = o/m, j = o % m;
            if(i+1 == n || j+1 == m || i == 0 || j == 0) {b = false;}
            if(i+1 != n && board[i+1][j] == 'O' && !v[m*(i+1) + j]) {q.push(m*(i+1) + j); s.insert(m*(i+1) + j); v[m*(i+1) + j] = 1;}
            if(j+1 != m && board[i][j+1] == 'O' && !v[m*i + j + 1]) {q.push(m*i + j + 1); s.insert(m*i + j + 1); v[m*i + j + 1] = 1;}
            if(i != 0 && board[i-1][j] == 'O' && !v[m*(i-1) + j]) {q.push(m*(i-1) + j); s.insert(m*(i-1) + j); v[m*(i-1) + j] = 1;}
            if(j != 0 && board[i][j-1] == 'O' && !v[m*i + j - 1]) {q.push(m*i + j - 1); s.insert(m*i + j - 1); v[m*i + j - 1] = 1;} 
        }
        if(b) {
            for(int x : s) {
                board[x/m][x % m] = 'X';
            }
        }
        s.clear();
    }

    void solve(vector<vector<char>>& board) {
        int n = board.size(); int m = board[0].size();
        for(int i = 0; i < n; i++) {
            for(int j = 0; j < m; j++) {
                if(!v[m*i + j] && board[i][j] == 'O') bfs(m*i + j, board);
            }
        }
    }
};
20558