LeetCode Weekly Contest 25 之 507.Perfect Number

LeetCode Weekly Contest 25

赛题

本次周赛主要分为以下4道题：

507 Perfect Number (3分)

537 Complex Number Multiplication (6分)

545 boundary of Binary Tree (8分)

546 Remove Boxes (9分)

507 Perfect Number

Problem:

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)

没有什么难度的题，思路在题目中已经给你了，无非是在遍历条件上做点思考。

My first solution(14ms)

public boolean checkPerfectNumber(int num) {

//边界条件
if(num == 0 || num == 1) return false;

//第二部分
int sum = 1;
for (int i = 2; i * i <= num; i++) {
if (num % i == 0){
sum += i + num / i;
}
}

return sum == num;
}

注意一些细节，Perfect Number不能由它自己组成，所以排除0和1。num由两个相乘的数分解，所以遍历时只需要遍历num−−−−√个数即可，所以遍历条件为

i*i <= num

，注意等于号。