<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:
    int dir[4][2] = {{0,-1}, {-1,0}, {1,0}, {0,1}};

    bool dfs(int i, int j, vector<vector<bool>> &vis, vector<vector<char>> &board){
        vis[i][j] = 1;
        bool ans = 1;
        for(auto k: dir){
            int x = i + k[0];
            int y = j + k[1];
            if(x < 0 || y < 0 || x >= board.size() || y >= board[0].size()){
                ans = 0;
                continue;
            }
            if(board[x][y] == 'X' || vis[x][y] == 1) continue;
            ans = ans & dfs(x, y, vis, board);
        }
        return ans;
    }

    void overwrite(int i, int j, vector<vector<bool>> &vis, vector<vector<char>> &board){
        vis[i][j] = 1;
        board[i][j] = 'X';
        for(auto k: dir){
            int x = i + k[0];
            int y = j + k[1];
            if(x < 0 || y < 0 || x >= board.size() || y >= board[0].size()) continue;
            if(board[x][y] == 'X' || vis[x][y] == 1) continue;
            overwrite(x, y, vis, board);
        }
    }

    void solve(vector<vector<char>>& board) {
        int n = board.size(), m = board[0].size();
        vector<vector<bool>> vis(n, vector<bool>(m, 0));
        vector<vector<bool>> vis2(n, vector<bool>(m, 0));

        for(int i = 0; i < n; i++){
            for(int j = 0; j < m; j++){
                bool ans = 0;
                if(vis[i][j] == 0 && board[i][j] == 'O') ans = dfs(i, j, vis, board);
                if(ans) overwrite(i, j, vis2, board);
            }
        }
    }
};
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