507. 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 class Solution {  
    public boolean checkPerfectNumber(int num) {  
        if (num == 1) {  
            return false;  
        }  
        int sum = 1, sqrt = (int)Math.sqrt(num);  
        for (int i = 2; i <= sqrt; i ++) {  
            if (num % i == 0) {  
                sum += i + (i * i == num? 0: num / i);  
            }  
        }  
        return sum == num;  
    }  
}