leetcode笔记（八）263. Ugly Number

• 题目描述

Write a program to check whether a given number is an ugly number.

Ugly numbers are positive numbers whose prime factors only include 2, 3, 5.

Example 1:

Input: 6
Output: true
Explanation: 6 = 2 × 3

Example 2:

Input: 8
Output: true
Explanation: 8 = 2 × 2 × 2

Example 3:

Input: 14
Output: false 
Explanation: 14 is not ugly since it includes another prime factor 7.

Note:

1. 1 is typically treated as an ugly number.
2. Input is within the 32-bit signed integer range: [−231,  231 − 1].

• 解题思路

虽然这道题属于简单类型，但是我小学数学不太好，质因数的概念并不是特别清楚，查了下百度百科才有了思路。

所以，也在这儿记录下。另外，自己实现的算法也不是特别好，也正好把自己的和讨论区的实现对比下～

• 我的代码

class Solution {
public:
    bool isUgly(int num) {
        if(num <= 0)
            return false;
        
        while(num != 1)
        {
            if(num % 2 == 0)
            {
                num = num / 2;
            }
            else if(num % 3 == 0)
            {
                num = num / 3;
            }
            else if(num % 5 == 0)
            {
                num = num / 5;
            }
            // other prime factor
            else
            {
                return false;
            }
        }
        
        return true;
    }
};

 

• 更快的代码

// 这个代码，减少了判断，性能就高了～确实也没必要判断。
// 对问题理解不深刻，无法认识应用隐含条件。
class Solution {
public:
    bool isUgly(int num) {
        if(num == 0) return false;
        while(num % 2 == 0) num /= 2;
        while(num % 3 == 0) num /= 3;
        while(num % 5 == 0) num /= 5;
        if(num > 1) return false;
        if(num == 1) return true;
        return false;
    }
};