507. Perfect Number.md

507. Perfect Number

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Difficulty : Easy

Related Topics : Math

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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)

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Solution

• mine
  • Java
    • Runtime: 2 ms, faster than 71.27%, Memory Usage: 38.2 MB, less than 6.52% of Java online submissions

      // O(N^(1/2))time
      // O(1)space
      public boolean checkPerfectNumber(int num) {
          if(num < 2) return false;
          int res = 1;
          int max = (int)Math.sqrt(num) + 1;
          for(int i = 2;  i < max; i++){
              if(num % i == 0){
                  res += i + num / i;
              }
          }
          return res == num;
      }
      

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