없는 숫자 더하기(문제36)

import java.util.*;

class Solution {
    public int solution(int[] numbers) {
        Set<Integer> set = new HashSet();
        for(int n : numbers){
            set.add(n);
        }
        
        int answer = 0;
        for(int i=1;i<=9;i++){
            if(!set.contains(i)) answer += i;
        }
        return answer;
    }
}