数列求和

import java.util.*;
public class Main{
 public static void main(String[] args){
     Scanner sc = new Scanner(System.in);
     int n = sc.nextInt();
     double f = 0d;
     double sum = 0d;
     for(int i = 1; i <= n; i++){
         f = 1d / (5 * (2 * i - 1)) - 1d / (5 * 2 * i);
         sum += f;
     }
     System.out.printf("%.4f", sum);
 }
}

回文素数的个数

import java.util.*;
public class Main{
 static Map<String, Boolean> map;
 public static void main(String[] args){
     Scanner sc = new Scanner(System.in);
     int N = sc.nextInt();
     int M = sc.nextInt();
     int ans = 0;
     map = new HashMap();
     for(int i = Math.max(10, N); i <= M; i++) {
         String s = i + "";
         for(int j = 0; j < s.length(); j++) {
             String b = s.substring(0, j) + s.substring(j + 1);
             if(is(b)) {
                 ans++;
                 break;
             }
         }
     }
     System.out.print(ans);
 }

 public static boolean is(String s) {
     if(!map.containsKey(s)){
         s = Integer.parseInt(s) + "";
         if(s.equals(new StringBuilder(s).reverse().toString())) {
             int n = Integer.parseInt(s);
             boolean hui = true;
             if(n == 0 || n == 1) {hui = false;}
             else {
                 for(int i = 2; i < n; i++) {
                     if(n / i * i == n) {
                         hui = false;
                         break;
                     }
                 }
             }
             map.put(s, hui);
         }else {
             map.put(s, false);
         }
     }
     return map.get(s);
 }
}