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Rotting Oranges LC #994 Medium Graph · Multi-Source BFS
Problem

Given an m×n grid: 0=empty, 1=fresh, 2=rotten. Every minute, fresh oranges 4-adjacent to rotten become rotten. Return minimum minutes until no fresh remains, or -1 if impossible.

Example
Input: [[2,1,1],[1,1,0],[0,1,1]]
Output: 4
Try Examples
Custom: rows separated by / (0=empty 1=fresh 2=rotten)
Approach
Multi-Source BFS
Enqueue ALL rotten oranges initially. Each BFS level = 1 minute. Spread rot to fresh neighbors. Return minutes if fresh==0, else -1.
Grid — 0=empty 1=fresh 2=rotten
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Variables
Minute
—
Fresh
—
Queue
—
New rot
—
Step Logic
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Ready
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Algorithm
1
Count fresh, enqueue all rotten
2
BFS: each level = 1 minute, rot spreads to neighbors
3
Return minutes if fresh==0, else -1
Time
O(m·n)
Space
O(m·n)
Why Multi-Source BFS

All rotten oranges start simultaneously. Each BFS level = 1 minute. Fresh neighbors become rotten when visited. If fresh>0 after BFS ends, some oranges are unreachable → return -1.