507. Perfect Number

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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: 28Output: TrueExplanation: 28 = 1 + 2 + 4 + 7 + 14

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

题意：给一个正数，判断他的因子的和加起来是不是和他相等，相等则为完美数，返回true，否则返回false

思路：这题关键是要想到用math.sqrt()求根公式，一开始我是直接去循环num，结果超时。

class Solution {    public boolean checkPerfectNumber(int num) {        if(num == 1){            return false;        }        int sum = 1;        int n = (int)Math.sqrt(num);        for(int i=2;i<= n;i++){            if(num%i==0){                sum += i + (i * i == num? 0: num / i);              }        }        if(sum == num){            return true;        }else{            return false;        }    }}