Odd Man Out Solution:

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class Kattis {
    public static void main(String[] args) throws Exception{
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
        int cases = Integer.parseInt(reader.readLine()) + 1;
        StringBuilder sb = new StringBuilder("");
        for(int i = 1; i < cases; i++){
            int guests = Integer.parseInt(reader.readLine());
            List<Integer> check = new ArrayList<>();
            List<Integer> invNums = Arrays.stream(reader.readLine().split(" ")).map(Integer::valueOf).collect(Collectors.toList());
            for(int j: invNums){
                if(check.contains(j))
                    check.remove((Integer)j);
                else
                    check.add(j);
            }
            sb.append("Case #" + i + ": " + check.get(0) + System.lineSeparator());
        }
        System.out.print(sb);
    }
}