1007. 素数对猜想 (20)

 1 import java.util.*;
 2 
 3 public class Main {
 4     private static Map<Integer,Boolean> map=new HashMap<Integer,Boolean>();
 5     private static boolean isPrime(int num){
 6         if(map.get(num)!=null)return map.get(num);
 7         for(int i=2;i<=Math.sqrt(num);i++){
 8             if(num%i==0) {
 9                 map.put(num,false);
10                 return false;
11             }
12         }
13         map.put(num,true);
14         return true;
15     }
16 
17 
18     public static void main(String[] args) {
19         Scanner in = new Scanner(System.in);
20         int size=0;
21         while (in.hasNext()) {
22             int N=in.nextInt();
23             int count=0;
24             for(int i=5;i<=N;i+=2){
25                if(isPrime(i)&&isPrime(i-2)){
26                    count++;
27                }
28             }
29             System.out.println(count);
30 
31         }
32     }
33 }