507. Perfect Number （数学）

https://leetcode.com/problems/perfect-number/discuss/

题目：求完美数

代码一：直接判断

class Solution {public:    bool checkPerfectNumber(int num) {        int sum=1;        for(int x=2;x<=sqrt(num);x++)            if(num%x==0) sum+=x,sum+=num/x;        return (sum==num&&num!=1);    }};

代码二：打表

class Solution {public:    bool checkPerfectNumber(int num) {        static unordered_set<int> n = {6, 28, 496, 8128, 33550336};        return n.count(num);    }};