<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:

    def validate(self, board, visited, points, i, j, m, n):
        visited[i][j] = True
        points.add((i, j))

        if i == 0 or i == m-1 or j == 0 or j == n-1:
            is_surrounded = False
        else:
            is_surrounded = True

        # Check edge: up
        if i == 0:
            is_surrounded = False
        elif visited[i-1][j]:
            pass
        elif board[i-1][j] == "O":
            is_surrounded &= self.validate(board, visited, points, i-1, j, m, n)

        # Check edge: down
        if i == m-1:
            is_surrounded = False
        elif visited[i+1][j]:
            pass
        elif board[i+1][j] == "O":
            is_surrounded &= self.validate(board, visited, points, i+1, j, m, n)

        # Check edge: left
        if j == 0:
            is_surrounded = False
        elif visited[i][j-1]:
            pass
        elif board[i][j-1] == "O":
            is_surrounded &= self.validate(board, visited, points, i, j-1, m, n)

        # Check edge: right
        if j == n-1:
            is_surrounded = False
        elif visited[i][j+1]:
            pass
        elif board[i][j+1] == "O":
            is_surrounded &= self.validate(board, visited, points, i, j+1, m, n)

        return is_surrounded

    def solve(self, board: List[List[str]]) -> None:
        """
        Do not return anything, modify board in-place instead.
        """
        m = len(board)
        n = len(board[0])
        visited = [[False for _ in range(n)] for __ in range(m)]

        for i in range(m):
            for j in range(n):
                if visited[i][j]:
                    continue
                if board[i][j] == "O":
                    points = set()
                    if self.validate(board, visited, points, i, j, m, n):
                        for (x, y) in points:
                            board[x][y] = "X"
                visited[i][j] = True

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