Create 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

思路：检测所有边界上的O，并将其入queue，将其值改为1。
           从queue中取点，检查领节点，如果为O则改为1，同时入queue。
          当queue为空时遍历board，将余下的O置为X，1置为O。