Perfect Number
A perfect number is a positive integer that is equal to the sum of its positive divisors, excluding the number itself. A divisor of an integer x is an integer that can divide x evenly.
Given an integer n, return true if n is a perfect number, otherwise return false.
Example 1:
Input: num = 28
Output: true
Explanation: 28 = 1 + 2 + 4 + 7 + 14
1, 2, 4, 7, and 14 are all divisors of 28.
Example 2:
Input: num = 7
Output: false
Constraints:
1 <= num <= 108
思路一：枚举，遍历 [1, n] 的数
public boolean checkPerfectNumber(int num) {
int sum = 1;
int z = num / 2;
for (int i = 2; i < z; i++) {
int x = num % i;
if (x == 0) {
sum += i;
z = num / i;
if (z != x) sum += z;
}
}
return sum == num;
}