<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:
    void travel(vector<vector<char>>& board, vector<vector<int>>& visited, int& curr, int& flag, int i, int j)
    {
        if(i<0 || i>=board.size() || j<0 || j>=board[0].size())
        {
            flag = 1; // marking that the region got safe
            return;
        }
        if(visited[i][j]==curr || board[i][j]!='O')
            return;
        visited[i][j] = curr;
        travel(board, visited, curr, flag, i-1, j); // up
        travel(board, visited, curr, flag, i+1, j); // down
        travel(board, visited, curr, flag, i, j-1); // left
        travel(board, visited, curr, flag, i, j+1); // right
        return;
    }
    void solve(vector<vector<char>>& board) 
    {
        // visit all the 0 regions
        // if while doing so, we come across an edge, then that region is safe
        // if we don't come across any edge during our travel, then that region is captured
        vector<vector<int>> visited(board.size(), vector<int>(board[0].size(), 0));
        unordered_set<int> regions; // to store the regions being eaten
        int curr = 1; // to store the current region
        int flag = 0; // to decide whether to capture a region or not
        for(int i=0;i<board.size();i++)
        {
            for(int j=0;j<board[0].size();j++)
            {
                if(board[i][j]=='O' && visited[i][j]==0)
                {
                    travel(board, visited, curr, flag, i, j);
                    cout<<flag<<curr<<endl;
                    if(flag==1)
                        regions.insert(curr);
                    curr++;
                    flag = 0;
                }   
            }
        }

        for(int i=0;i<board.size();i++)
        {
            for(int j=0;j<board[0].size();j++)
            {
                // change all the unprotected "O" to "X"
                if(board[i][j]=='O' && regions.find(visited[i][j])==regions.end())
                    board[i][j]='X';
            }
        }
        return ;
        
    }
};
20558