LeetCode 994 Rotting Oranges - Problem Statement

You have a grid where each cell contains a fresh orange (value $1$), a rotten orange (value $2$), or is empty (value $0$). Every minute, any fresh orange adjacent to a rotten orange (up, down, left, right) becomes rotten.

Return the minimum number of minutes that must elapse until no cell has a fresh orange. If this is impossible, return $-1$.

This is multi-source BFS. You start from all rotten oranges at once, not from a single source. Each rotten orange spreads rot in parallel. Before reading on, think: how would you track time? What makes this different from single-source BFS?