leetcode507: Perfect Number

We define the Perfect Number is a positive integer that is equal to the sum of all its positive divisors except itself.
Now, given an integer n,
write a function that returns true when it is a perfect number and false when it is not.

Example:

Input: 28
Output: True
Explanation: 28 = 1 + 2 + 4 + 7 + 14

Note: The input number n will not exceed 100,000,000. (1e8)

public boolean checkPerfectNumber(int num) {
int sum = 1;
if (num == 1)
return false;
for (int i = 2; i < Math.sqrt(num); i++) {
if (num % i == 0)
sum = sum + i + num / i;
}
return sum == num;
}