<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 {
private:
    int ROWS;
    int COLS;
    void BFS(vector<vector<char>>& board, int row, int col) {
        queue<pair<int, int>> q;
        q.push(make_pair(row, col));
        while (q.size() > 0) {
            auto front = q.front();
            int row = front.first, col = front.second;
            q.pop();
            if (board[row][col] != 'O') continue;
            board[row][col] = 'E';
            if (col < COLS - 1) q.push(make_pair(row, col + 1));
            if (row < ROWS - 1) q.push(make_pair(row + 1, col));
            if (col > 0) q.push(make_pair(row, col - 1));
            if (row > 0) q.push(make_pair(row - 1, col));
        }
    }

public:
    void solve(vector<vector<char>>& board) {
        int numRows = board.size();
        if (numRows == 0) return;
        int numCols = board[0].size();
        this->ROWS = numRows;
        this->COLS = numCols;
        vector<pair<int, int>> border;
        for (int r = 0; r < this->ROWS; ++r) {
            border.push_back(make_pair(r, 0));
            border.push_back(make_pair(r, this->COLS - 1));
        }
        for (int c = 0; c < this->COLS; ++c) {
            border.push_back(make_pair(0, c));
            border.push_back(make_pair(this->ROWS - 1, c));
        }
        for (pair<int, int> border_pair : border)
            this->BFS(board, border_pair.first, border_pair.second);
        for (int r = 0; r < this->ROWS; ++r) {
            for (int c = 0; c < this->COLS; ++c) {
                if (board[r][c] == 'O') board[r][c] = 'X';
                if (board[r][c] == 'E') board[r][c] = 'O';
            }
        }
    }
};
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