<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:
    static constexpr int dr[] = {-1, +1, 0, 0};
    static constexpr int dc[] = {0, 0, -1, +1};
    int n, m;
    int vis[202][202], vis2[202][202];
    
    bool isValidCell(int r, int c) {
        return ( 0 <= r && r < n && 0 <= c && c < m );
    }
    
    bool isNonRegionCell(vector<vector<char>>& board, int ur, int uc) {
        bool res = false;
        vis[ur][uc] = 1;
        
        for(int i = 0; i < 4; i++) {
            int vr = ur + dr[i];
            int vc = uc + dc[i];
            if(!isValidCell(vr, vc)) {
                res = true;
                continue;
            }
            
            if(board[vr][vc] == 'O' && !vis[vr][vc])
                res |= isNonRegionCell(board, vr, vc);
        }
        
        return res;
    }
    
    void floodfill(vector<vector<char>>& board, int ur, int uc) {
        vis2[ur][uc] = 1;
        board[ur][uc] = 'X';
        
        for(int i = 0; i < 4; i++) {
            int vr = ur + dr[i];
            int vc = uc + dc[i];
            // if(!isValidCell(vr, vc)) continue;
            if(board[vr][vc] == 'O' && !vis2[vr][vc])
                floodfill(board, vr, vc);
        }
    }
    
public:
    void solve(vector<vector<char>>& board) {
        n = board.size();
        m = board[0].size();
        
        // memset(vis, 0, sizeof vis);
        // memset(vis2, 0, sizeof vis2);
        
        for(int i = 0; i < n; i++) {
            for(int j = 0; j < m; j++) {
                if(board[i][j] == 'O' && !vis[i][j]) {
                    if(!isNonRegionCell(board, i, j))
                        floodfill(board, i, j);
                }
            }
        }
        
        // for(int i = 0; i < n; i++) {
        //     for(int j = 0; j < m; j++)
        //         cout << board[i][j] << " ";
        //     cout << "\n";
        // }
            
    }
};
17971