<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, grid: List[List[int]]) -> None:

        # idea
        # do not do anything if the position is on boundary or if it is X (captured)
        # only modify if the position is "O" inside


        # --- the idea same as number of closed regions would not work here. We need to keep the boundary "O" safe from getting flipped

        # all the regions on the boundary and also which are connected to the boundary, we need to mark them => T (temporary)
        # then we can modify the inside regions to X if they are 'O'
        # at the end, flip all T to O => these are the nodes who were connected to a boundary region O

        if not grid:
            return
        
        def markBorderConnectedSafe(row, col):

            if row < 0 or row >= len(grid) or col < 0 or col >= len(grid[0]) or grid[row][col] != 'O':
                return

            grid[row][col] = 'T' # capture the region
            markBorderConnectedSafe(row + 1, col)
            markBorderConnectedSafe(row - 1, col)
            markBorderConnectedSafe(row, col + 1)
            markBorderConnectedSafe(row, col - 1)

        # 1. mark all boundary and boundary connected 'O' as 'T'
        for row in [0, len(grid)-1]:
            for col in range(len(grid[0])):
                if grid[row][col] == 'O': 
                    markBorderConnectedSafe(row, col)
        
        for row in range(len(grid)):
            for col in [0, len(grid[0])-1]:
                if grid[row][col] == 'O':
                    markBorderConnectedSafe(row, col)
    

        # 2. capture the regions => mark inside regions as X
        for row in range(1, len(grid)-1):
            for col in range(1, len(grid[0])-1):
                if grid[row][col] == 'O':
                    grid[row][col] = 'X'

        # 3. now, we need to flip the boundary regions that we marked with 'T'
        for row in range(len(grid)):
            for col in range(len(grid[0])):
                if grid[row][col] == 'T':
                    grid[row][col] = 'O'


        # WOULD NOT WORK
        # if not grid:
        #     return

        # def captureIsland(row, col):
            
        #     # either on boundary or outside
        #     if (row <= 0 or row >= len(grid)-1 or col <= 0 or col >= len(grid[0])- 1):
        #         return
            
        #     # either this is surrounded or already visited
        #     if grid[row][col] == "X":
        #         return

        #     grid[row][col] = "X" # capture the region

        #     up = captureIsland(row - 1, col)
        #     down = captureIsland(row + 1, col)
        #     right = captureIsland(row, col + 1)
        #     left = captureIsland(row, col - 1)
        
        # # excluding boundaries for 
        # for i in range(1, len(grid)):
        #     for j in range(1, len(grid[0])):
        #         if grid[i][j] == "O":
        #             captureIsland(i, j)
        
17390