Trace the example grid.
Initial: queue = [(0,0)], fresh = 6.
Minute 1: (0,0) rots neighbors. (0,1) and (1,0) rot. Queue = [(0,1), (1,0)]. Fresh = 4.
Minute 2: (0,1) rots (0,2). (1,0) rots (1,1). Fresh = 2.
Minute 3: (0,2) has no fresh neighbors. (1,1) rots (2,1). Fresh = 1.
Minute 4: (2,1) rots (2,2). Fresh = 0.
Queue empty. Fresh = 0. Return 4.
Each cell processed at most once. time. space for queue.