<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;
    vector<vector<char>> *v;

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

    void dfs(int si, int sj) {
        vis[si][sj] = true;
        (*v)[si][sj] = 'X';

        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 dfs2(int si, int sj) {
        vis[si][sj] = true;

        for (int i = 0; i < 4; i++) {
            int ci = si + fr[i].first;
            int cj = sj + fr[i].second;
            if (valid(ci, cj))
                dfs2(ci, cj);
        }
    }

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

        vector<int> _i = {0, n - 1}, _j = {0, m - 1};
        for (int i : _i)
            for (int j = 0; j < m; j++)
                if (valid(i, j))
                    dfs2(i, j);
        for (int j : _j)
            for (int i = 0; i < n; i++)
                if (valid(i, j))
                    dfs2(i, j);

        for (int i = 1; i < n - 1; i++)
            for (int j = 1; j < m - 1; j++)
                if (valid(i, j))
                    dfs(i, j);
    }
};
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