Problem
A perfect number is a positive integer that is equal to the sum of its positive divisors, excluding the number itself. A divisor of an integer x is an integer that can divide x evenly.
Given an integer n, return true** if n is a perfect number, otherwise return **false.
Example 1:
Input: num = 28
Output: true
Explanation: 28 = 1 + 2 + 4 + 7 + 14
1, 2, 4, 7, and 14 are all divisors of 28.
Example 2:
Input: num = 7
Output: false
Constraints:
1 <= num <= 10^8
Solution (Java)
class Solution {
public boolean checkPerfectNumber(int num) {
int s = 1;
for (int sq = (int) Math.sqrt(num); sq > 1; sq--) {
if (num % sq == 0) {
s += sq + (num / sq);
}
}
return num != 1 && s == num;
}
}
Explain:
nope.
Complexity:
- Time complexity : O(n).
- Space complexity : O(n).