Question
A CDN cache-invalidation wave spreads across a grid of edge servers. Each cell is 0 (no server), 1 (server holding stale content), or 2 (server already invalidated, a source). Each minute, every invalidated server pushes the invalidation to its orthogonally adjacent stale servers. Return the number of minutes until no stale server remains, or -1 if some stale server can never be reached. If there are no stale servers, the answer is 0.
time_to_saturate(grid: list[list[int]]) → int[[[2,1,1],[1,1,0],[0,1,1]]]out4State your approach and its time/space complexity out loud before you optimize. Handle the edge cases (empty input, duplicates, overflow), and say why you chose this over the brute force. Green tests are the floor, not the grade.
Vibe coding: describe the solution in plain language (or narrate it) and the coach grades your approach. Generating runnable code from your description is coming next.