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g0101_0200.s0130_surrounded_regions.readme.md Maven / Gradle / Ivy

130\. Surrounded Regions

Medium

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.

**Example 1:**

![](https://assets.leetcode.com/uploads/2021/02/19/xogrid.jpg)

**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"]]

**Constraints:**

*   `m == board.length`
*   `n == board[i].length`
*   `1 <= m, n <= 200`
*   `board[i][j]` is `'X'` or `'O'`.