[String]507. Perfect Number

题目：507. Perfect Number

判断一个正数是否是perfect number，即它是否可以等于自身所有正因数（除了自己）之和。

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)

注意：
1是false。
分解因式只用判断到它的平方根即可。sqrt（num）

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