<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 solve(vector<vector<char>>& board) {
        int m = board.size(), 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 (i == 3 && j == 3)   print(visited);
                if (visited[i][j] == 0 && board[i][j] == 'O'){
                    if (dfs1(i, j, board, visited)) dfs2(i, j, board, visited);
                    else dfs3(i, j, board, visited);
                }
                if (i == 0 && j == 0)   print(visited);
            }
        }
    }

    void print (vector<vector<int>> &visited){
        for (int i = 0; i < visited.size(); i++){
            for (int j = 0; j < visited[0].size(); j++){
                cout << visited[i][j] << " ";
            }
            cout << endl;
        }
        cout << endl;
    }

    // checks existence of a valid surroundable region
    bool dfs1 (int i, int j, vector<vector<char>> &board, vector<vector<int>> &visited){
        int m = board.size(), n = board[0].size();
        if (i < 0 || i == m || j < 0 || j == n)     return 0;
        if (visited[i][j] == 1 || board[i][j] == 'X')    return 1;
        visited[i][j] = 1;
        if (!dfs1(i-1, j, board, visited) || !dfs1(i+1, j, board, visited) || !dfs1(i, j+1, board, visited) || !dfs1(i, j-1, board, visited))    return 0;
        return 1;
    }

    //converts the surroundable region from 'O' to 'X'
    void dfs2 (int i, int j, vector<vector<char>> &board, vector<vector<int>> &visited){
        int m = board.size(), n = board[0].size();
        if (i < 0 || i == m || j < 0 || j == n) return;
        if (visited[i][j] == 2 || board[i][j] == 'X') return;
        visited[i][j] == 2;
        board[i][j] = 'X';
        dfs2(i-1, j, board, visited);
        dfs2(i+1, j, board, visited);
        dfs2(i, j+1, board, visited);
        dfs2(i, j-1, board, visited);
    }

    //converts the surrounded region visited values from 1 to 2 to make it inaccessable
    void dfs3(int i, int j, vector<vector<char>> &board, vector<vector<int>> &visited){
        int m = board.size(), n = board[0].size();
        if (i < 0 || i == m || j < 0 || j == n) return;
        if (visited[i][j] == 2 || board[i][j] == 'X') return;
        visited[i][j] = 2;
        dfs3(i-1, j, board, visited);
        dfs3(i+1, j, board, visited);
        dfs3(i, j+1, board, visited);
        dfs3(i, j-1, board, visited);
    }
};
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