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)

 

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;
                if(num/i != i) sum += num/i;
            }
        }
        return sum+1 == num;
    }
}

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