Concepts4

SPFA is a queue-based optimization of Bellman–Ford that only relaxes edges from vertices whose distance just improved.

Breadth-First Search (BFS) explores a graph level by level, visiting all vertices at distance d from the source before any at distance d+1.

Multi-source BFS explores an unweighted graph starting from several sources at once to compute the minimum distance to any source for every vertex.

A queue is a First-In-First-Out (FIFO) line where you add at the back and remove from the front in O(1) time.