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)

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

首先O（n）肯定不行，那就像求最大公约数那样求到根号下，然后加起来看等不等于原来的num