LeetCode Weekly Contest 25 之 507.Perfect Number

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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−−−−√\sqrt {num}个数即可，所以遍历条件为i*i <= num，注意等于号。