<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:
    const static int N = 205;
    bool vis[N][N];
    vector<pair<int, int>> fr = {{0, 1}, {0, -1}, {-1, 0}, {1, 0}};

    int n, m;
    bool corner;
    vector<vector<char>> v;
    vector<pair<int, int>> tr;

    bool valid(int i, int j) {
        return !(i < 0 || i >= n || j < 0 || j >= m || v[i][j] == 'X' || vis[i][j] == true);
    }

    bool is_corner(int i, int j) {
        return (i == 0 || i == n - 1 || j == 0 || j == m - 1);
    }

    void dfs(int si, int sj) {
        vis[si][sj] = true;
        corner = corner || is_corner(si, sj);
        tr.push_back({si, sj});

        for (int i = 0; i < 4; i++) {
            int ci = si + fr[i].first;
            int cj = sj + fr[i].second;
            if (valid(ci, cj)) dfs(ci, cj);
        }
    }

    void solve(vector<vector<char>> &board) {
        memset(vis, false, sizeof(vis));
        v = board;
        n = v.size(), m = v.front().size();

        for (int i = 0; i < n; i++)
            for (int j = 0; j < m; j++) {
                if (valid(i, j)) {
                    corner = false;
                    tr = {};
                    dfs(i, j);
                    if (!corner)
                        for (pair<int, int> pr : tr) board[pr.first][pr.second] = 'X';
                }
            }
    }
};
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