[LeetCode]65. Ugly Number丑数

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. For example, 6, 8 are ugly while 14 is not ugly since it includes another prime factor 7.

Note that 1 is typically treated as an ugly number.

Credits:
Special thanks to @jianchao.li.fighter for adding this problem and creating all test cases.

 

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解法：丑数被定义为其所有的质因子只有2、3和5的正数，另外1也是丑数。因此对输入正数将其质因子2、3、5都去掉，看看结果是否是1就可以了。

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

另一种写法：

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

第一种写法效率更高。