[LeetCode] 263. Ugly Number 丑陋数

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].

这道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，参见代码如下：

解法一：

class Solution {
public:
    bool isUgly(int num) {
        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 num == 1;
    }
};

我们也可以换一种写法，分别不停的除以 2，3，5，并且看最后剩下来的数字是否为1即可，参见代码如下：

解法二：

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;
        return num == 1;
    }
};