java题解 | #完全数计算#

import java.util.*;

public class Main{

public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    while(sc.hasNext()){
        int n = sc.nextInt();
        int count = 0;
        //6是最小完全数
        for (int i = 6; i <= n; i++) {
            // 求约数只需要遍历到开方即可
            int val = (int)Math.sqrt(i);
            int sum = 1;
            for (int j = 2; j<=val; j++) {
                if (i%j == 0) {
                    // 把一对约数都加进去
                    sum+=j;
                    sum+=i/j;
                }
            }
            if (sum == i) {
                count++;
            }
        }
        System.out.println(count);
    }
    sc.close();

} }