The junction's group

A town has n junctions joined by one-way streets. A street (u, v) lets you drive from u to v, but not back. Return the strongly connected group of junction s: every junction v such that you can drive from s to v and also from v back to s, perhaps along many streets. List them in increasing order. Junction s is always in its own group.

streets = [(0, 1), (1, 4), (4, 0), (5, 4), (5, 6),
           (1, 2), (2, 3), (3, 7), (7, 6), (6, 2)]
group_of(8, streets, 0)   # [0, 1, 4]
group_of(8, streets, 5)   # [5]
group_of(8, streets, 6)   # [2, 3, 6, 7]

This is the town of lesson 1’s figure, with A to H as 0 to 7. A street may lead from a junction to itself, and the same street may be listed twice.

Limits

• Up to 100,000 junctions and 200,000 streets.
• A search may go 100,000 streets deep, so recursion is too deep. The big tests have 3 seconds each.