Trace the 3×3 grid.
Start: (0,0), length 1.
Level 1 (length 2): (0,1), (1,0)? (1,0) is blocked. Only (0,1).
Level 2 (length 3): from (0,1), reach (0,2), (1,2).
Level 3 (length 4): from (0,2), (1,2), reach... (1,2) can reach (2,2)!
Actually: (0,0) → (0,1) → (0,2) → (1,2) → (2,2) is 5 cells.
Wait, let me retrace. Level 4 (length 5): from (1,2), reach (2,2). Return 5.
Each cell visited once. time. space for queue.