RosettaCodeData/Task/Find-the-missing-permutation/Mathematica/find-the-missing-permutation.math

ProvidedSet = {"ABCD" , "CABD" , "ACDB" , "DACB" , "BCDA" , "ACBD",
"ADCB" , "CDAB", "DABC", "BCAD" , "CADB", "CDBA" , "CBAD" , "ABDC",
"ADBC" , "BDCA",  "DCBA" , "BACD", "BADC", "BDAC" , "CBDA", "DBCA", "DCAB"};

Complement[StringJoin /@ Permutations@Characters@First@#, #] &@ProvidedSet


->{"DBAC"}