请考虑性能
import java.util.*; public class Main{ public static void main(String[] args){ Scanner sc=new Scanner(System.in); int n=Integer.valueOf(sc.next()); if(n<=1) System.out.println(0); else if(n==2) System.out.println(1); else{ int ans=1; for(int i=3;i<=n;i+=2){ if(isSu(i)) ans++; } System.out.println(ans); } } public static boolean isSu(int num){ for(int i=2;i<=Math.sqrt(num);i++){ if(num%i==0) return false; } return true; } }