# [Tutor] Fwd: Graph question was: Re: (no subject)

Roel Schroeven roel at roelschroeven.net
Mon Mar 22 14:58:54 EDT 2021

```Alan Gauld via Tutor schreef op 16/03/2021 om 1:34:
> On 15/03/2021 23:56, Alan Gauld wrote:

>
> I assume this should be written:
>
> 3
> 4
> 0,1
> 1,2
> 2,0
> 0,2
> 2
>
> ie 3 nodes, 4 edges and the mystery node is 2.

That is indeed almost certainly the case. IIRC problems on HackerRank,
and possibly other sites like it, use an input format like that.

> The solution is apparently 0,1 but I'm not sure why?

I think I understand it. We have a set of directed edges i -> j. Then we
are given a node k, find all edges that go towards that node k, and
finally enumerate all starting nodes of those edges.

In the example, we have 4 edges:
- 0 -> 1
- 1 -> 2
- 2 -> 0
- 0 -> 2
We are asked to find all edges that end in 2 (because k is given as 2).
Those are:
- 1 -> 2
- 0 -> 2
The starting nodes therefore are 1 and 0 (or 0 and 1; it looks like the
order doesn't matter).

Or in code: [s for (s, j) in edges if j == k]

--
"Honest criticism is hard to take, particularly from a relative, a
friend, an acquaintance, or a stranger."
-- Franklin P. Jones

Roel Schroeven

```