<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(int row, int col, vector<vector<char>>& board,
        vector<vector<bool>>&visited)
    {
        if(row < 0 || col < 0 || row >= board.size() || 
            col >= board[0].size() || visited[row][col] == true
            || board[row][col] == 'X')
        {
            return;
        }
        visited[row][col] = true;
        DFS(row - 1, col, board, visited);
        DFS(row + 1, col, board, visited);
        DFS(row, col - 1, board, visited);
        DFS(row, col + 1, board, visited);
    }
    void boundaryCheck(int startRow, int maxRow, int startCol, int maxCol, vector<vector<char>>& board, vector<vector<bool>>&visited)
    {
        for(int i = startRow; i < maxRow; i++){
            for(int j = startCol; j < maxCol; j++){
                if(board[i][j] == 'O' && visited[i][j] == false){
                    DFS(i, j, board, visited);
                }
            }
        }
    }
    void solve(vector<vector<char>>& board) {
        int n = board.size();
        int m = board[0].size();
        vector<vector<bool>>visited(n, vector<bool>(m, false));
        boundaryCheck(0, 1, 0, m, board, visited);
        boundaryCheck(0, n, 0, 1, board, visited);
        boundaryCheck(n - 1, n, 0, m, board, visited);
        boundaryCheck(0, n, m-1, m, board, visited);
        for(int i = 0; i < n; i++){
            for(int j = 0; j < m; j++){
                if(board[i][j] == 'O' && visited[i][j] == false){
                    board[i][j] = 'X';
                }
            }
        }
    }
};
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