leetcode/java/507_Perfect_Number.java at master · jakehoare/leetcode · GitHub

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https://leetcode.com/problems/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.

Try all divisors from 2 to sqrt(n).

Time - O(sqrt(n))

Space - O(1)

class Solution {

public boolean checkPerfectNumber(int num) {

if (num == 1) // no divisors apart from self

return false;

int sumDivisors = 1;

int maxDivisor = (int) Math.sqrt(num);

for (int i = 2; i <= maxDivisor; ++i) {

if (num % i == 0)

sumDivisors += i + num/i;

}

if (maxDivisor * maxDivisor == num) // double-counted if num is a square

sumDivisors -= maxDivisor;

return sumDivisors == num;

}

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