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)
class Solution { public boolean checkPerfectNumber(int num) { int sqr = (int) Math.sqrt(num); if(num == 1) return false; int res = 1; for(int i = 2; i <= sqr; i++){ if(num % i == 0){ res = res + i + num / i; } } return res == num; } }
首先O(n)肯定不行,那就像求最大公约数那样求到根号下,然后加起来看等不等于原来的num