# 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:

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: Surrounded regions should not be on the border, which means that any ‘O’ on the border of the board are not flipped to ‘X’. Any ‘O’ that is not on the border and it is not connected to an ‘O’ on the border will be flipped to ‘X’. Two cells are connected if they are adjacent cells connected horizontally or vertically.

## 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’.

# Solution: DFS

## 效能

### Complexity

• Time Complexity: O(N), where N is the number of cells in the board. In the worst case where it contains only the O cells on the board.
• Space Complexity: O(N), where N is the number of cells in the board.

### LeetCode Result

• Runtime: 15 ms
• Memory Usage: 9.9 MB