<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:
    unordered_set<int> surround_mark;
    int dir[4][2] = {0, 1, 1, 0, 0, -1, -1, 0};
    int count = 0;
    void solve(vector<vector<char>>& board) {
        int m = board.size();
        int n = board[0].size();
        vector<vector<int>> visited(m, vector<int>(n, 0));
        for(int i=0;i<m;i++){
            for(int j=0;j<n;j++){
                if(board[i][j]=='O' && visited[i][j]==0){
                    count+=1;
                    visited[i][j] = count;
                    dfs(board, visited, i, j);
                }
            }
        }
        for(int i=0;i<m;i++){
            for(int j=0;j<n;j++){
                if(visited[i][j]!=0 && surround_mark.count(visited[i][j])==0){
                    board[i][j] = 'X';
                }
            }
        }
    }
    void dfs(vector<vector<char>>& board, vector<vector<int>>& visited, int x, int y){
        int m = board.size();
        int n = board[0].size();

        for(int i=0;i<4;i++){
            int next_x = x+dir[i][0];
            int next_y = y+dir[i][1];
            if(next_x<0 || next_x >=m || next_y <0 || next_y >= n){
                surround_mark.insert(count);
                continue;
            }
            else{
                if(board[next_x][next_y]=='O' && visited[next_x][next_y]==0){
                    visited[next_x][next_y] = count;
                    dfs(board, visited, next_x, next_y);
                }
            }
        }
    }
};
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