public static int lonelyinteger(List<Integer> a) {
    // Write your code here
        Collections.sort(a);
        for(int i = 0; i < a.size(); i += 2){
            if( i == a.size() -1 || a.get(i) != a.get(i + 1))
                return a.get(i);
        }
        return 0;
    }