LeetCode Perfect Number

原题链接在这里：https://leetcode.com/problems/perfect-number/#/description

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)

题解：

把每个divisor相加看是否等于num.

Time Complexity: O(Math.sqrt(num)). Space: O(1).

AC Java:

 public class Solution {
     public boolean checkPerfectNumber(int num) {
         if(num == 1){
             return false;
         }

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