leetcode: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)

题意&解题思路

求所给数的质因子之和是否和该数相等。暴力求解，从2到(√n)。把符合条件的都加起来即可。

class Solution {
public:
bool checkPerfectNumber(int num) {
if(num == 1)return false;
int ans = 1;
int x = sqrt(num);
for(int i = 2; i <= x; i++){
if(num % i == 0){
ans += i;
ans += num/i;
}
}
return ans == num;
}
};