Surrounded Regions
题目
Given a 2D board containing 'X' and 'O', capture all regions surrounded by 'X'.
A region is captured by flipping all 'O's into 'X's in that surrounded region.
For example,
X X X X
X O O X
X X O X
X O X X
After running your function, the board should be:
X X X X
X X X X
X X X X
X O X X
思路
从边界的 'O' 开始，广度优先搜索，可以搜索到的 'O'，就保留下来。
注意：
- 不要用深度优先搜索，会递归深度过深
- 记录是否访问过
