Each sightseeing route is a sequence of road numbers y1, …, yk, k ≥ 2

in fact k>2 && k!=2

first I have to deal with the case when k=2, but I got WA.
then I ignore this case, and got AC.

maybe it's not important to somebodyt.
but still it's a little error.

It's very important detail and it's not a LITTLE error!!!
I wonder why such an important detail wasn't corrected so long. That "detail" costed me 5 submissions with WA#1 :((
Damn it! :-E

it seems to me, thats your error, not the incorrect problem.

if case k==2 is impossible, does not matter, deal you with it or not.

yes, if input is
2 2
1 2 1
2 1 1
-1

then output should be

No solution.

TRUE!
I was also misled by the description, but now I get AC ignoring cases where k=2.