507. Perfect Number

• Difficulty: Easy

• Topics: Math

• Similar Questions:

  • Self Dividing Numbers

Problem:

We define the Perfect Number is a positive integer that is equal to the sum of all its positive divisors except itself.

Example:

Input: 28
Output: True
Explanation: 28 = 1 + 2 + 4 + 7 + 14

Note: The input number n will not exceed 100,000,000. (1e8)

Solutions:

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

        if (root * root == num) {
            sum -= root;
        }

        return sum == num;
    }
};