507. Perfect Number

题目：

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)

思路：

本题思路比较简单，构造从2开始的循环，循环次数小于等于sqrt(num)，期间求num得约数

代码：

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