1
 2
 3
 4
 5
 6
 7
 8
 9
10
11

public int minNumberDisappeared (int[] nums) {
        HashSet<Integer> set = new HashSet<>();
        for (int i = 0; i < nums.length; i++) {
            set.add(nums[i]);
        }
        int res = 1;
        while (set.contains(res)) {
            res++;
        }
        return res;
    }