PerfectNumber.java

package com.fishercoder.solutions;

/**
 * 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)

 */
public class PerfectNumber {

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

}