perfect-number

Perfect Number – LeetCode 507

Problem

Description

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

Answer

Original

Code

class Solution {
public:
    bool checkPerfectNumber(int num) {
        if(num == 1) return false;
        for(int i = sqrt(num), n = num; i >= 2; --i){
            if(n % i == 0){
                num -= n / i;                
                num -= i;
                if(num < 0) return false;
            }
        }
        return num == 1;
    }
};

思路

看题说话。时间复杂度$O(\sqrt{n})$，空间复杂度$O(1)$。
耗时$6$ ms，排名$18.63\%$

Better

思路

还没看到更好的解法。