<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.
        """
        # Flood fill the islands forwards, push to a stack, then
        # convert all to Xs if not safe. 
        # Keep track of visited nodes. 
        rows = len(board)
        cols = len(board[0])
        self.visited = [[False for _ in range(cols)] for _ in range(rows)]
        self.regionStack = []
        self.regionSafe = False


        def isBoundary(row, col, board):
            if row == 0 or row == len(board) -1 or col == 0 or col == len(board[0]) - 1:
                return True
            else:
                return False


        def floodFillSafe(row, col, board):
            # Check out of bounds
            if row < 0 or row > len(board) - 1 or col < 0 or col > len(board[0]) - 1:
                return

            if self.visited[row][col] or board[row][col] != "O":
                return

            self.visited[row][col] = True
            self.regionStack.append((row,col))
            
            # Check boundary
            if row == 0 or row == len(board) -1 or col == 0 or col == len(board[0]) - 1:
                self.regionSafe = True

            for dRow, dCol in [(0,-1), (0,1), (1, 0), (-1, 0)]:
                floodFillSafe(row + dRow, col + dCol, board)

                

        for i in range(rows):
            for j in range(cols):
                if not self.visited[i][j] and board[i][j] == "O":
                    floodFillSafe(i, j, board)
                    if not self.regionSafe:
                        for row, col in self.regionStack:
                            board[row][col] =  "X"
                    self.regionStack = []
                    self.regionSafe = False





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