268. Missing Number

class Solution {
    public int missingNumber(int[] nums) {
        Arrays.sort(nums);
        int length = nums.length + 1;
        for (int i = 0; i < length; i++) {
            if (i == nums.length || nums[i] != i) {
                return i;
            }
        }
        return -1;
    }
}

class Solution {
    public int missingNumber(int[] nums) {
        Set<Integer> set = new HashSet();
        for (int num : nums) {
            set.add(num);
        }
        for (int i = 0; i < nums.length + 1; i++) {
            if (!set.contains(i)) {
                return i;
            }
        }
        return -1;
    }
}