The Graduate by Buck Henry, based on the novel by Charles Webb. Calder Willingham was also credited.
From the 2008 Vanity Fair article The Making of The Graduate by Sam Kashner:
[Mike] Nichols couldn’t have been more pleased with the final screenplay, which he credits entirely to Henry, though Calder Willingham ended up with first billing. “I didn’t even know there were other scripts until I was finished,” recalls Henry, “but Willingham sued for credit, and won. I was stunned at first, but it’s interesting, because from that moment on, I never gave a shit about credit. Give me the money, credit whom you want. And in some cases, I don’t really want my name on the movie!”