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23. After teaming with mega-producer Max Martin (the man behind some of Britney Spears, Kelly Clarkson and Katy Perry's biggest hits) for his Ariana Grande duet "Love Me Harder," he rewrote his verse. "It was a great song," he allowed to Rolling Stone, "but it was a little generic. I couldn't hear myself on it. So I changed it and made it dark." Fortunately Martin liked his edits and the two joined forces for 2015's Beauty Behind the Madness. His official pop breakthrough, it featured collaborations with Lana Del Rey and Ed Sheeran and went triple platinum.