Use iteration instead of recursion in Graphs.hasCycle.

[copybara-service]

Use iteration instead of recursion in Graphs.hasCycle.

This can avoid stack overflows for graphs with long paths.

RELNOTES=graph: Improved Graphs.hasCycle to avoid causing StackOverflowError for long paths.

[perceptron8]

I'm happy and slightly disappointed at the same time! ;) Have you considered #6059 instead? Main loop feels a little bit simpler plus it avoids successors copying (on the other hand, successors are visited in reverse order, which obviously doesn't change the result, but is worth noting). WDYT?

[cpovirk]

Ouch, sorry.

@jrtom, any thoughts? You may recall from 2 hours ago that I described my own approach as "clumsy," and I don't think you went out of your way to disagree... :)

[perceptron8]

I've just realized that in #6059 successors are actually "copied" (by pushing them one by one to the stack - that's the source of mentioned reverse order), which makes those PRs more similar than I thought. As for simplicity, let's rely on @jrtom taste.

[jrtom]

@perceptron8 I had completely forgotten about your previous offer; my apologies. 😅

I think I slightly prefer cpovirk@'s version mostly because it retains the abstraction of the subgraphHasCycle method (which makes it, to me, a little easier to read) but I don't think that the difference matters a great deal; they are pretty similar and I believe that they both do the job.

On that basis I'd suggest we go with the solution that we already have committed, but @perceptron8 I really appreciate your attention to this issue and your desire to make this code base as good as it can be. Thank you! 😃