LeetCode 507 Perfect Number（完美数字）

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)

题目大意：给出一个正整数，判断它是不是完美数。完美数定义：如果一个正整数N的所有因子之和等于N，那么N就是一个完美数字。

代码如下：

bool checkPerfectNumber(int num) {
if(num == 1) return false;
int n = sqrt(num);
int ans = 1;
for(int i = 2;i <= n;i++){
if(num % i == 0){
ans += i + num / i;
}
}
return ans == num;
}