Rotting Oranges (Multi-source BFS)

Each cell of the grid is empty (0), a fresh orange (1) or a rotten orange (2). Every minute, every fresh orange next to (up/down/left/right) a rotten one becomes rotten. How many minutes until no fresh orange is left? Print -1 if that never happens.

Input:

• line 1: r c
• next r lines: c values each

Output: the number of minutes, or -1.

Constraints: 1 ≤ r, c ≤ 300.

Think about it: put all rotten oranges in the queue at time 0 (multi-source BFS) and spread level by level — each level is one minute. If no orange is fresh at the start, the answer is 0.