<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 {
    const int UNLANDED = -1;
public:
    void solve(vector<vector<char>>& board) {
        n = board.size();
        m = board[0].size();
        if (n <= 1)
            return;
        if (m <= 1)
            return;
        lands = vector<vector<int>>(n, vector<int>(m, UNLANDED));

        for (int i = 0; i<n; i++)
            for (int j = 0; j<m; j++)
            {
                markLands(board, i, j);
                if (borderLands.find(lands[i][j]) == borderLands.end())
                    board[i][j] = 'X';
            }
    }

private:
    vector<vector<int>> lands;
    set<int> borderLands;
    int n = -1;
    int m = -1;
    int landId = 0;

    void markLands(const vector<vector<char>>& board, int i, int j)
    {
        if (lands[i][j] == UNLANDED)
        {
            markLands(board, landId, i, j);
            landId++;
        }
    }

    void markLands(const vector<vector<char>>& board, int id, int i, int j)
    {
        if (i < 0)
            return;
        if (j < 0)
            return;
        if (i >= n)
            return;
        if (j >= m)
            return;
        if (lands[i][j] != UNLANDED)
            return;
        if (board[i][j] != 'O')
            return;
        lands[i][j] = id;
        if (i == 0 || j == 0 || i == n-1 || j == m-1)
            borderLands.insert(id);
        markLands(board, id, i+1, j);
        markLands(board, id, i, j+1);
        markLands(board, id, i-1, j);
        markLands(board, id, i, j-1);
    }

};
26596