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##### ###### ##### ### # # ### # # ###### ## ## ## ## ## ## ## # # # # # ## ##### #### ##### # # # # # # # #### ## # ## ## ## ## # # # # # ## ## # ###### ## ### # ### # ######
Walkthrough and analysis
Trace the example board.
Border 'O's: only (3,1) is 'O' on the border (left edge).
DFS from (3,1). Mark as 'S'. No neighbors are 'O'.
Board after marking:
X X X X
X O O X
X X O X
X S X X
Flip remaining 'O's to 'X':
X X X X
X X X X
X X X X
X S X X
Change 'S' back to 'O':
X X X X
X X X X
X X X X
X O X X
Border scan: . DFS: . Final passes: . Total: .