[Level 1] 소수 찾기

class Solution {
    public boolean[] prime;
    
    public int solution(int n) {
        int answer = 0;
        prime = new boolean[n+1];
        
        checkPrime(n);
        for(int i=1; i<=n; i++) {
            if(!prime[i]) { answer++; }
        }
        
        return answer;
    }
    
    public void checkPrime(int n) {
        prime[0] = true; prime[1] = true;
        
        for(int i=2; i*i<=n; i++) {
            if(!prime[i]) {
                for(int j=i*i; j<=n; j+=i) { prime[j] = true; }
            }
        }
    }
}