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)

Solution：

思路：
Time Complexity: O(N) Space Complexity: O(N)

Solution Code：

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