##### ###### ##### ### # # ### # # ###### ## ## ## ## ## ## ## # # # # # ## ##### #### ##### # # # # # # # #### ## # ## ## ## ## # # # # # ## ## # ###### ## ### # ### # ######
##### ###### ##### ### # # ### # # ###### ## ## ## ## ## ## ## # # # # # ## ##### #### ##### # # # # # # # #### ## # ## ## ## ## # # # # # ## ## # ###### ## ### # ### # ######
The core idea
Instead of finding surrounded regions, find unsurrounded regions and protect them.
DFS/BFS from all 'O's on the border. Mark these as "safe" (temporarily change to 'S' or another marker).
After marking, all remaining 'O's are surrounded. Flip them to 'X'.
Finally, change 'S' back to 'O'.
