263. Ugly Number 丑数

题目链接
tag:

• Easy；

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

思路：
  本题让我们检测一个数是否为丑陋数，所谓丑陋数就是其质数因子只能是2,3,5。那么最直接的办法就是不停的除以这些质数，如果剩余的数字是1的话就是丑陋数了，有两种写法，如下所示：

C++ 解法一：

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

C++ 解法二：

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