Zac Efron -- Post-Rehab Reunion With 'Hairspray' Cast

Efron appeared in great spirits as he hit up the Griffith Park Haunted Hayride with Julianne Hough and former "Hairspray" co-star Brittany Snow ... as well as director Adam Shankman.

TMZ broke the story -- the 25-year old actor completed 2 rounds of treatment earlier this year to combat the cocaine problem that took over his life.

Julianne's brother Derek Hough snapped a pic with the group sharing hugs and smiles -- and posted it online afterward saying, "Great time last night with great friends."