Missing Number

Java

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; 
		}
}