_507.java

package com.fishercoder.solutions;

/**
 * 507. Perfect Number
 *
 * 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 _507 {

    public static class Solution1 {
        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;
        }
    }
}