<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 dfs(vector<vector<char>>& board, int row, int col, vector<vector<char>> &ans,vector<vector<bool>> &vis){
        vis[row][col] = true;

        vector<int> delrow{0,1,0,-1};
        vector<int> delcol{-1,0,1,0};

        for(int i=0;i<4;i++){
            int nrow = row + delrow[i];
            int ncol = col + delcol[i];

            if(nrow >= 0 && nrow < board.size() && ncol >=0 && ncol < board[0].size() && !vis[nrow][ncol] && board[nrow][ncol] == 'O'){
                ans[nrow][ncol] = 'O';
                dfs(board, nrow,ncol,ans,vis);
            }
        }
    }
    void solve(vector<vector<char>>& board) {
        int n = board.size();
        int m = board[0].size();

        vector<vector<char>> ans(n, vector<char>(m, 'X'));
        vector<vector<bool>> vis(n, vector<bool>(m, false));
        // first row
        int col = m-1;
        while(col >= 0){
            if(board[0][col] == 'O'){
                ans[0][col] = 'O';
                vis[0][col] = true;
                dfs(board,0,col,ans,vis);
            }
            col--;
        }
        //last row
        col = m-1;
        while(col >= 0){
            if(board[n-1][col] == 'O'){
                ans[n-1][col] = 'O';
                vis[n-1][col] = true;
                dfs(board,n-1,col,ans,vis);
            }
            col--;
        }
        // first col
        int row = n-1;
        while(row >= 0){
            if(board[row][0] == 'O'){
                ans[row][0] = 'O';
                vis[row][0] = true;
                dfs(board,row,0,ans, vis);
            }
            row--;
        }
        // last col
        row = n-1;
        while(row >= 0){
            if(board[row][m-1] == 'O'){
                ans[row][m-1] = 'O';
                vis[row][m-1] = true;
                dfs(board,row,m-1,ans, vis);
            }
            row--;
        }
        board = ans;
    }
};
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