<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 solve(self, board: List[List[str]]) -> None:
        """
        Do not return anything, modify board in-place instead.
        """
        m = len(board)
        n = len(board[0])
        visited = set()
        r_diff = [-1, 1, 0, 0]
        c_diff = [0, 0, -1, 1]

        def dfs(board, start):
            stack = [start]
            result = [start]
            while stack:
                i, j = stack.pop()
                if (i, j) not in visited:
                    visited.add((i, j))
                    for k in range(4):
                        dx = i + r_diff[k]
                        dy = j + c_diff[k]
                        if dx >= 0 and dy >= 0 and dx < m and dy < n and (dx, dy) not in visited and board[dx][dy] == "O":
                            result.append((dx, dy))
                            stack.append((dx, dy))
            return result

        for col in range(n):
            if board[0][col] == "O":
                board[0][col] = "-"
                cells = dfs(board, (0, col))
                for x,y in cells:
                    board[x][y] = "-"
            if board[m-1][col] == "O":
                board[m-1][col] = "-"
                cells = dfs(board, (m-1, col))
                for x,y in cells:
                    board[x][y] = "-"

        for row in range(1, m-1):  
            if board[row][0] == "O":
                board[row][0] = "-"
                cells = dfs(board, (row, 0))
                for x,y in cells:
                    board[x][y] = "-"
            if board[row][n-1] == "O":
                board[row][n-1] = "-"     
                cells = dfs(board, (row, n-1))
                for x,y in cells:
                    board[x][y] = "-"

        for i in range(m):
            for j in range(n):
                if board[i][j] == "O" and (i, j) not in visited:
                    cells = dfs(board, (i, j))
                    for c in cells:
                        x, y = c
                        board[x][y] = "X"

        for i in range(m):
            for j in range(n):
                if board[i][j] == "-":
                    board[i][j] = "O"
        return 
29990