# Breadth-First Search - A single FIFO queue, storing vertices at distance $k$ and then $k+1$. - Enqueue every white vertex exactly once, blacken them when they are dequeued. - $O(E + V)$. - Breadth-first [[tree]], also determines a [[shortest-path]] on unweighted graphs. _Predecessor subgraph_ $G_\pi = (V_\pi, E_\pi)$ - Defined as starting from one source only.