Set<Integer> set = new HashSet<>(a);
        
        Iterator<Integer> it = set.iterator();
        
        int result = 0;
        
        while(it.hasNext()) {
            int next = it.next();
            int fr = Collections.frequency(a, next);
            if (fr == 1) {
                result = next;
                break;
            }
        }
        
        return result;