Solution.java

class FirstUnique {
    private Map<Integer, Integer> cnt = new HashMap<>();
    private Set<Integer> unique = new LinkedHashSet<>();

    public FirstUnique(int[] nums) {
        for (int v : nums) {
            cnt.put(v, cnt.getOrDefault(v, 0) + 1);
        }
        for (int v : nums) {
            if (cnt.get(v) == 1) {
                unique.add(v);
            }
        }
    }

    public int showFirstUnique() {
        return unique.isEmpty() ? -1 : unique.iterator().next();
    }

    public void add(int value) {
        cnt.put(value, cnt.getOrDefault(value, 0) + 1);
        if (cnt.get(value) == 1) {
            unique.add(value);
        } else {
            unique.remove(value);
        }
    }
}

/**
 * Your FirstUnique object will be instantiated and called as such:
 * FirstUnique obj = new FirstUnique(nums);
 * int param_1 = obj.showFirstUnique();
 * obj.add(value);
 */