<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 {
    int xx[4] = {-1,0,1,0};
    int yy[4] = {0,-1,0,1};

    void dfs(int i, int j,
        int curr_comp,
        vector<vector<char>>& board,
        vector<vector<int>>& comp,
        unordered_map<int,bool>& region){
        
        int n = board.size();
        int m = board[0].size();

        comp[i][j] = curr_comp;

        if(i ==0 || j==0 || i == n-1 || j == m-1){
            region[curr_comp] = true;
        }

        for(int k=0;k<4;k++){
            int nx = xx[k] + i;
            int ny = yy[k] + j;

            if(nx<0 || nx >= n || ny<0 || ny>=m){
                continue;
            }

            if(comp[nx][ny] == -1 && board[nx][ny] == 'O'){
                dfs(nx,ny,curr_comp,board,comp,region);
            }
        }

    }

public:
    void solve(vector<vector<char>>& board) {
        int n = board.size(); int m = board[0].size();

        unordered_map<int,bool> region;

        vector<vector<int>> comp(n, vector<int>(m, -1));

        int n_comp = 0;

        for(int i=0;i<n;i++){
            for(int j=0;j<m;j++){
                if(comp[i][j] == -1 && board[i][j] == 'O'){
                    dfs(i,j,n_comp,board,comp,region);
                    n_comp++;
                }
            }
        }

        for(int i=0;i<n;i++){
            for(int j=0;j<m;j++){
                if(board[i][j] == 'O'){
                    if(!region[comp[i][j]]){
                        board[i][j] = 'X';
                    }
                }
            }
        }
    }
};
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