[Leetcode] 507. Perfect Number 解题报告

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：

哈哈，练手题目，注意特殊情况。另外循环只可以到sqrt(num)，否则会超时。

代码：

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