题解 | #完全数计算#

方法一：暴力算法
Scanner scanner = new Scanner(System.in);
int n = scanner.nextInt();
int count = 0;
for (int i = 2; i <= n; i++) {
    int sum = 0;
    for (int j = 1; j < i; j++) {
        if (i % j == 0) {
            sum += j;
        }
    }
    if (sum == i) {
        count++;
    }
}
System.out.println(count);
scanner.close();
        
 

方法二：优化算法
Scanner scanner = new Scanner(System.in);
int n = scanner.nextInt();
int count = 0;
for (int i = 2; i <= n; i++) {
    int sum = 1;
    for (int j = 2; j * j <= i; j++) {
        if (i % j == 0 && i % j != j) {
            //短路与，不整除直接过
            //遍历到根号j即可
            sum = sum + j + i / j;
        } else if (i % j == j) {
            sum += j;
        }
    }
    if (sum == i) {
        count++;
    }
}
System.out.println(count);
scanner.close();