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)

完数 = 他所有正除数（不包括他本身）之和

判断一个数是不是完数

C++（3ms）：

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

java(15ms):

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