<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:
        """
        from every "O", do a dfs to its neighboring "O"s
            if we ever reach the edge in the dfs, then it's not surrounded

        [["X","O","X","O","O","O","O"]
         ["X","O","O","O","O","O","O"]
         ["X","O","O","O","O","X","O"]
         ["O","O","O","O","X","O","X"]
         ["O","X","O","O","O","O","O"]
         ["O","O","O","O","O","O","O"]
         ["O","X","O","O","O","O","O"]]
        """
        visited = set()
        m, n = len(board), len(board[0])

        def get_neighbors(curr):
            dirs = [(0, 1), (0, -1), (1, 0), (-1, 0)]
            neibs = []
            for dx, dy in dirs:
                nx, ny = curr[0] + dx, curr[1] + dy
                if 0 <= nx < m and 0 <= ny < n and board[nx][ny] == "O":
                    neibs.append((nx, ny))
            return neibs

        def dfs(curr, path):
            stack = [curr]
            flag = True
            while stack:
                curr = stack.pop()
                visited.add(curr)
                if curr[0] in [0, m-1] or curr[1] in [0, n-1]:
                    flag = False
                path.append(curr)
                neibs = get_neighbors(curr)
                for neib in neibs:
                    if neib not in visited:
                        stack.append(neib)
            if flag: 
                return path
            return []
            
        to_change = []
        for i in range(m):
            for j in range(n):
                if board[i][j] == "O" and (i, j) not in visited:
                    res = dfs((i, j), [])
                    if res:
                        to_change += res
        for x, y in to_change:
            board[x][y] = "X"



        
25850