codility - Counting Elements (MissingInteger)

    public int solution(int[] A) {
        // write your code in Java SE 8
 
        HashSet<Integer> hs = new HashSet<>();
 
        for (int a : A)
            hs.add(a);
 
        int v = 0;
        for (int i = 0; i < A.length; i++) {
            v++;
            if (hs.remove(v))
                continue;
            else
                return v;
 
        }
 
        return ++v;
 
    }