can i use Junktroller on opponent graveyard
can i use Junktroller on opponent graveyard
Yes. Junktroller's ability says "a graveyard", and that's exactly what it means. You can put a card from any player's graveyard on the bottom of its owner's library (which will always be the same player whose graveyard you took the card from, since a player's graveyard can only ever contain their own cards).