1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
class Solution {
    public int solution(int n) {
        int answer = 0;
        int[] arr = new int[n+1];

        for(int i=0;i<=n;i++){
            arr[i]=i;
        }//0=1 n-1 =n이 된다
        arr[1]=0;
        for(int i=2;i<=n;i++){//2부터 계산해줌
            if(arr[i]==0)continue;
            for(int j=i+i;j<=n;j+=i){
                arr[j]=0;
            }
        }


        for(int i=0;i<arr.length;i++){
            if(arr[i]!=0){
                answer++;
            }
        }
        return answer;
    }
}