I'll gladly let you have all the set theory and epsilon-delta

int rvdn(int n, char* string, int stringlen) { for(int i = 0; i < stringlen; i++) for(int j = i; j < stringlen; j++) if(string[i] == string[j]) return 0; return 1; }
rvdn str = (sort str) == (nub $ sort str)
rvdn :: (Eq a) => [a] -> Bool -- to keep the monomorphism restriction from biting you rvdn = any (\x -> (head x) `elem` (tail x)) . init . tails
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