You are given an m x n board of characters X and O . Capture every region of O s that is completely surrounded by X s by flipping those O s to X . An O is safe (never flipped) if it can reach the border of the board by moving up, down, left, or right through other O s. Modify the board in place . Input format The input is a single argument [board] , a JSON 2-D array of single-character strings, e.g. [[[ X , X , X ],[ X , O , X ],[ X , X , X ]]] . The graded answer is the board after your method mutates it; any value your method returns is ignored.