Given an?m x n?matrix?board?containing?'X'?and?'O',?capture all regions that are 4-directionally?surrounded by?'X'.

A region is?captured?by flipping all?'O's into?'X's in that surrounded region.

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Example 1:

Input: board = [["X","X","X","X"],["X","O","O","X"],["X","X","O","X"],["X","O","X","X"]]Output: [["X","X","X","X"],["X","X","X","X"],["X","X","X","X"],["X","O","X","X"]]Explanation: Notice that an 'O' should not be flipped if: - It is on the border, or - It is adjacent to an 'O' that should not be flipped. The bottom 'O' is on the border, so it is not flipped. The other three 'O' form a surrounded region, so they are flipped.

Example 2:

Input: board = [["X"]]Output: [["X"]]

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Constraints:

• m == board.length

• n == board[i].length

• 1 <= m, n <= 200

• board[i][j]?is?'X'?or?'O'.

典型的DFS先將邊上的為O的改成#，然后dfs跟他相關(guān)聯(lián)的位置也一并改成#，剩下的O就是唄X包圍著的，然后2個(gè)for循環(huán)就可以了。

Runtime:?4 ms, faster than?63.84%?of?Java?online submissions for?Surrounded Regions.

Memory Usage:?52.7 MB, less than?13.82%?of?Java?online submissions for?Surrounded Regions.

Next challenges:

Walls and Gates