code: import java.util.ArrayList;
  
  import com.google.common.base.Joiner;
  import com.google.common.collect.ImmutableSet;
  import com.google.common.collect.Lists;
  
  public class FindMissingPermutation {
  	public static void main(String[] args) {
  		Joiner joiner = Joiner.on("").skipNulls();
  		ImmutableSet<String> s = ImmutableSet.of("ABCD", "CABD", "ACDB",
  				"DACB", "BCDA", "ACBD", "ADCB", "CDAB", "DABC", "BCAD", "CADB",
  				"CDBA", "CBAD", "ABDC", "ADBC", "BDCA", "DCBA", "BACD", "BADC",
  				"BDAC", "CBDA", "DBCA", "DCAB");
  
  		for (ArrayList<Character> cs : Utils.Permutations(Lists.newArrayList(
  				'A', 'B', 'C', 'D')))
  			if (!s.contains(joiner.join(cs)))
  				System.out.println(joiner.join(cs));
  	}
  }
  