##### ###### ##### ### # # ### # # ###### ## ## ## ## ## ## ## # # # # # ## ##### #### ##### # # # # # # # #### ## # ## ## ## ## # # # # # ## ## # ###### ## ### # ### # ######
##### ###### ##### ### # # ### # # ###### ## ## ## ## ## ## ## # # # # # ## ##### #### ##### # # # # # # # #### ## # ## ## ## ## # # # # # ## ## # ###### ## ### # ### # ######
Walkthrough and analysis
Start BFS from "0000".
Level 0: {"0000"}.
Level 1: all 8 neighbors of "0000" (except deadends). "1000", "9000", "0100", "0900", etc.
Level 2: neighbors of level 1 states (not visited, not deadends).
...continue until "0202" is found at level 6.
At most states (0000 to 9999). Each state has 8 neighbors. = since bounded. Space: for visited set.
