Surrounded Regions

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
DFS from the edge, mark all 'O's to 'M'. Then scan the matrix again, 'M' cannot be surrounded regions. So change all other 'O's to 'X'. BUG FREE!

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