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; } }