 Two-pass strongly connected component algorithm

Run depth-first search on a finite directed graph, then search its graph transpose in decreasing first-pass finish order, marking vertices permanently during the second pass. Each second-pass search tree is one strongly connected component. The component finish-order property ensures that the next search cannot enter another still-unassigned component in the graph transpose; mutual reachability ensures that it reaches every vertex in its own component. Two traversals and transposition take $O(|V|+|E|)$ time.